whats the optimal sizing for a preflop 4bet?

as always, it depends
larger oop
w/e you do, don't go over 1/3 eff stacks vs competent opp unless you jam

cash or tournament?

one, two or three opponents?

Stack sizes?

History? Read? V type?

Position?

In a tournament everything is geared off of the 'current' opening size, which tends to get smaller the further into the structure things go. There are spots where a 4-bet of 2.1x may be plenty.

You also have to consider what you want the opponent to do .. or what you want the opponent to think you are going to do against their response. You may be able to 2x an opponent into a 'wanted' shove or you may want to 3.5x to make sure your opponent knows that you (or they) are pot committed.

I tend to agree that anything over 1/3 stack is a commitment to call off for stacks either PF or on most Flops. GL

Depends on stacksizes and ranges. I'm quite fond of very large (pot-sized) 4-bets in 100bb cash games, but most players tend to go nearer to 22-25bb, depending on positions and the amount of money of that's already in the pot.

im assuming your playing 100bb cash game. if so usually around just above 2x the 3-bet if your in position and just above 3x the 3-bet out of position.

What's the rationale behind a pot-sized 4bet?

usually ppl 4b in the 66-75%p region
but depends on pos and stack depth

This. Before you can answer the question, you have to start with answering this "why" question. Why are you considering a 4bet? Value? Bluff? Semi-Bluff? Isolation? Etc.

Then, like others have pointed out, the next step is to look at all the various factors, such as stack sizes, number of opponents, positions, game venue, etc.

When I revamped my pre-flop play with Snowie 3 or 4 years ago, I basically copied what it suggested. Large 4-bets mean you have more fold equity with your bluffs, and you get more money in the pot with your monsters.
With smaller sizes, the ranges should probably be a little bit different to what Snowie suggests. They also lead to tricky spots where you can get flatted and then have to play with an underpair and SPR of 1 or less. Hardly anyone flats a 32bb 4-bet. They usually jam or fold, which means I hardly ever have tricky post-flop spots in 4-bet pots. I probably lose some EV with my sizes, but I like easy decisions.

I like this reasoning, simpler strategies mean less mistakes.

I imagine you couldn't 4bet bluff much for that price though. Ranges would have to be tight. I've always used a smaller 4bet sizing, like 2.25x ish. It leaves the SPR low enough that I don't get pot committed, but tbh I've never even considered using a pot-sized 4!

there's a balance between pushing fold eq vs opp who 3b too much or over fold to 4bets and playing post with range adv vs those who don't
the hand doesn't end pre and you should prepare for that in advance. if you think about it, you're playing narrow ranges in a low spr pot, one of easier spots in poker, highly technical and dependent on knowing close to exact ranges to come up with the correct solution, but I find drilling 4b pot spots to be rather easy.
certainly easier compared to playing deep oop vs a competent opp in situations where equities run close, like in bvb srp 300x deep

bvb in particular it makes a lot of sense to 4b wide and small because how polarized bb 3bets (in theory). because bb uses all these junky offsuit hands to 3b, such as mid to low K/Q/Jx, random KXs, low suited gappers, that part of his range simply cannot defend to a small 4b.

always hitting pot to 4b is hardly the best way to approach this, as you're only making it easier for your opp. provided you study 4b pots, there are considerable edges to take advantage of. a lot people misplay in 4b pots, so take them to the part of the game tree you have studied and they hadn't.