The campaign I'm running has got a relatively large (6 players) Level 4 party. A few of them are excited about the Optional Class Features in Tasha's and I'm wondering how other DMs are handling adding these features in. Are you just adding them outright, are you making players choose between the original feature at that level or the new one, or are you leaving the new ones out? Of course I wanna follow what the players are excited about, but I don't want to accidentally OP the players, especially considering how the size of the party already makes encounter scaling somewhat difficult. 

I believe they are meant to be added directly to the existing class features.

For your particular situation, you might consider letting your players earn them as part of an appropriate quest like they would a magic item, or let them gain them at a rate of 1/week of downtime to represent additional training. Relatively easy, but not automatic.

I've told my players they're welcome to look the new features over, and given they're officially WotC endorsed, I'm allowing them to make the changes at their whims. However, I have also asked them to notify me of any changes they make, and if something is going to be dramatic enough to impact the narrative, I'll work with them to find a solution that fits. If they want to change subclasses however, I'll probably follow the guidelines that were given in the book.

I've allowed all of the options for new characters and I've gone back and retroactively allowed a single replacement to existing characters.  The caveat for pre-existing is they do not earn the new option until they have leveled up (all 4th level at about 4.6K point so it is not an immediate reward.) and spend 200gp for the training in a big city. 

To me, the players, Ranger, Rogue, Monk, and Wizard new options are not a problem or I see a power creep for these classes

I've allowed most of the class features. Ones I have disallowed are those worse than our house rules. For instance, I have been allowing a 1 cantrip change for the cleric upon each level-up. The Tasha rule is on the 4-level mark. That would be worse for the player,  so it is disallowed in favor of our house rule. Similarly I have been letting the sorcerer change Metamagic options upon level-up. Again, the "every 4 level" Tasha rule would be worse, so it is ignored.

I have stated that changing subclass or changing skill would require a substantial in-character justification. I understand that players may have thought skill A would be useful and decided skill B would be better, but character sheets are not just lumps of stats with no bearing on the gameplay. If your PC has been using History skill all along, and has even succeeded on some rolls based on proficiency, so that you are something of an in-character history nerd, and suddenly you forget all that history and now you are an expert at Arcana, this would make no sense in character. I'm not disallowing it outright, but I need a really good in character justification for it. Same with subclass change.

Additionally, I had told them that everything was on the table to be changed up to level 4, and then at level 4, whatever they have after level-up is now locked in. They all agreed to that. Yes, this was before Tasha, but if they had wanted to change subclass or skills in levels 1-4, it would have been allowed (one of them did, btw -- changed 1 skill and altered stat array allocation, at level 2). My reasoning is by level 4, you should have some handle on your PC. If you want to play something wildly different at that point, (a) wait for the next campaign, or (b) request a PC change, retire PC 1, and bring in PC 2. But no more major changes to the PC at this point.

Features that replace other ones, at least for my group, are not major IC immersion-breakers. The Ranger is actually sticking with most of the original Ranger abilities except swapping over to Favored Foe, and there is nothing IC needed for that since her Favored Enemy had not really come into play so far. She's keeping the rest of her options the same. Most of the other features can easily be explained IC as getting new stuff, just like leveling up, so again, no IC problem. It's the changing what has already been established, like say a Ranger switching from Beastmaster to Gloom Stalker, that I will not allow without major justification. (Again, they could have done that already in the first 4 levels -- now enough is enough and it is time to settle in and play the character you have.)