If I'm playing a monk assassin that get 2 attacks and 2 bonus attacks from flurry of blows and has the assassinate ability:

"Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit."

If I get the drop on the enemy party, and assuming that all 4 attacks hit, can I:

A:  deal 4 automatic criticals to one surprised creature using all 4 attacks?

and/or

B:  deal automatic criticals to 4 different creatures who haven't been attacked yet and are surprised?

Both of those are correct.

i would say neither...

i would say after first attack its not surprised anymore

and two you dont get 2 bonus actions.. you still only get one

flurry of blows means u use a ki point and get to do two unarmed attacks on your one bonus action

so couldnt split them up its one bonus action against one creature

Flurry of Blows

Immediately after you take the Attack action on your turn, you can spend 1 ki point to make two unarmed strikes as a bonus action.

also if your using weapons with your attack.. how are you suddle unarmed to do the flurry of blows?

For triggering the rogue’s Assassinate ability, when does a creature stop being surprised? After their turn in the round, or at the end of the round?

A surprised creature stops being surprised at the end of its first turn in combat.

From the Sage Advice Compendium.

This is also essentially stated in the Surprise rule itself, just not super clearly:

If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. A member of a group can be surprised even if the other members aren't.

So no matter what happens to you during other creature's turns, you're still surprised until the end of your own turn, so a creature should count as surprised no matter how many attacks you throw at it, as you're raining attacks upon it before it has a chance to react.

• When you use the Attack action with an unarmed strike or a monk weapon on your turn, you can make one unarmed strike as a bonus action. For example, if you take the Attack action and attack with a quarterstaff, you can also make an unarmed strike as a bonus action, assuming you haven’t already taken a bonus action this turn.

Having a monk weapon in one hand still leaves the other hand open for unarmed attacks.

Not to mention you can make unarmed attacks with your legs, head, elbows, teeth, etc.

I don't think those count for unarmed attacks.  Otherwise what's the point of saying that Loxodons and Simic Hybrids can use their trunks and extra appendages to make unarmed/grapple attacks?  That's one of their advantages is that they can hold weapons and/or shields in each hand and still perform unarmed/grapples attacks.

Quote from TDog84 >>

I don't think those count for unarmed attacks.  Otherwise what's the point of saying that Loxodons and Simic Hybrids can use their trunks and extra appendages to make unarmed/grapple attacks?  That's one of their advantages is that they can hold weapons and/or shields in each hand and still perform unarmed/grapples attacks.

Player's Handbook:

Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow

Unarmed strikes do not require a free hand. Grapple attempts do.