This is a beefed up specialized form of [feat]Polearm Master[/feat] which is already considered the best combat feat in the game. (Not to mention that whips are one handed finesse weapons too)
While it looks really cool not needing to spend a bonus action for the extra 1d4 of damage and turning all whips into whip +1's in your hands is a bit much.
To fit existing wording you could use:

• When you take the Attack action and attack with only a whip, you can use a bonus action to make a melee attack with another quick crack of the weapon. This attack uses the same ability modifier as the primary attack. The weapon’s damage die for this attack is a d4, and it deals the same damage type as the primary attack.

The last part is cool and in flavor for a whip, but there's no RAW for grappling with a weapon so there wouldn't be any rules to go by to give advantage for.

I wouldn't know how to really balance this without making it a carbon copy of PAM for whips or to just leave it as OP

Battlemaster's disarming attacks would be made with advantage to rip the weapon away with the whip. That's about it.

Hey there!

It is certainly a powerful feat, but I'd like to take a moment to talk about the intent of the feat and how you could modify it to suit a lower power game.

• You gain a +1 bonus to attack rolls you make with a whip.
  
  • This is pulled from the weapon feats UA, which is the general power level I tried to balance around.
  • This could easily be removed if you wanted to lower the power level without detracting from the core of the item.
• When you hit with a melee attack using a whip you deal an additional 1d4 damage of whatever type the whip deals.
  • The thought here is by spending a feat you are upgrading the damage of the whip to bring it in line with other martial melee weapons.
  • While there are certainly parallels, the extra attack from PAM is worth more than a single d4 in most cases as it allows you to add your damage bonuses and trigger on attack or on hit effects.
  • This can easily be changed to suit a lower power game. My suggestion would be to run is as upgrading the d4 damage the whip would normally do into a d6 rather than adding a d4. This gives some return on investment, still lets it come closer to competing with other weapon options, and gives the player a concrete benefit from their feat investment.
• While you are wielding a whip, other creatures provoke an opportunity attack from you when they enter your reach.
  • I envision this as filling a similar role to what it does on polearm master and gives a finesse option for people wanting to do that same kind of gameplay.
  • This could be cut to drop the power, but to me this is a core part of the feat.
• You have advantage on ability checks and attacks made to disarm or grapple enemies when using a whip as a part of the check or attack.
  • This one is a bit more on the fluff side, and by RAW doesn't necessarily do a ton. But with a bit of creativity and a DM willing to work with their player (which they have already started if they are using a homebrew feat!) it can create some interesting moments!
  • This is not necessary for the feat, but it is certainly fun and I have seen some creative applications so I encourage anybody reading this to give it a shot and play around with it a bit.

I hope that clears up a few things and gives you a direction if you wanted to modify it for your own game!

Thanks

I would add the benefit of proficiency with whip and maybe also make it a monk weapon. This would make whip usage more accessible to classes without proficiency in martial weapons, like rogue and monk.

I see alot of activity now? Could if be cause of FANDOM?

Slashing damage. The type of damage a whip does is slashing.

Under normal circumstances that would absolutely be the case, but I left it vague in case anybody wanted to use it with a magic weapon that might deal different damage types (i.e. a whip version of a sun blade).

I agree with null that this should be balanced with (and worded like) Polearm Master. 

A potential attack of opportunity when they enter your reach, and a bonus action d4 attack. I could maybe see the +1 to attack, just because it's a whip, but the advantage benefit is too strong.

+ well whips Are rarely used at the game, so I do not mind the feat to be a little bit stronger. 2d4 max damage is still fair because 1d8 have similar max damage. That +1 Is just for damage So who cares :D

+ That advantige might motivate player to do different things beside just attacking.

I really like this feat, thank you for having shared it!

Has anybody noticed that you also get one of the best parts of the sentinel feat, which is a really good feat. This feat is unbalanced and the only balancing is that the whip is a low damage dealing weapon.

Quote from Abelhawk >>

Slashing damage. The type of damage a whip does is slashing.

I'm pretty sure he meant it like if someone had homebrewed a chain or sword whip. From what I've heard it's done quite often.

The whip is such an underused combat option! Love the attention it's getting!!!

Quote from hi216 >>

Has anybody noticed that you also get one of the best parts of the sentinel feat, which is a really good feat. This feat is unbalanced and the only balancing is that the whip is a low damage dealing weapon.

 Polearm master gives you the approaching opportunity attack, not sentinel.

Quote from LunarSymphonia >>

Hey there!

It is certainly a powerful feat, but I'd like to take a moment to talk about the intent of the feat and how you could modify it to suit a lower power game.

• You gain a +1 bonus to attack rolls you make with a whip.
  
  • This is pulled from the weapon feats UA, which is the general power level I tried to balance around.
  • This could easily be removed if you wanted to lower the power level without detracting from the core of the item.
• When you hit with a melee attack using a whip you deal an additional 1d4 damage of whatever type the whip deals.
  • The thought here is by spending a feat you are upgrading the damage of the whip to bring it in line with other martial melee weapons.
  • While there are certainly parallels, the extra attack from PAM is worth more than a single d4 in most cases as it allows you to add your damage bonuses and trigger on attack or on hit effects.
  • This can easily be changed to suit a lower power game. My suggestion would be to run is as upgrading the d4 damage the whip would normally do into a d6 rather than adding a d4. This gives some return on investment, still lets it come closer to competing with other weapon options, and gives the player a concrete benefit from their feat investment.
• While you are wielding a whip, other creatures provoke an opportunity attack from you when they enter your reach.
  • I envision this as filling a similar role to what it does on polearm master and gives a finesse option for people wanting to do that same kind of gameplay.
  • This could be cut to drop the power, but to me this is a core part of the feat.
• You have advantage on ability checks and attacks made to disarm or grapple enemies when using a whip as a part of the check or attack.
  • This one is a bit more on the fluff side, and by RAW doesn't necessarily do a ton. But with a bit of creativity and a DM willing to work with their player (which they have already started if they are using a homebrew feat!) it can create some interesting moments!
  • This is not necessary for the feat, but it is certainly fun and I have seen some creative applications so I encourage anybody reading this to give it a shot and play around with it a bit.

I hope that clears up a few things and gives you a direction if you wanted to modify it for your own game!

Thanks

 I like this breakdown.

I'm playing a DEX/CON Whip barbarian. If you're willing to have a look at a potential homebrew feat before I post it, I would love to get some input. (gonna copy paste this request to a few others who had some good insights.)

I like this feat more then my DM does. The 2d4 damage per hit is good. Maybe one creature per round provokes an opportunity attack and you can replace one attack to disarm or trip a target with a DC save in place of damage.

well well well

"Whip it, whip it good!" - Devo