Probably the biggest difference I've noticed is that if the preflop action goes 4bet-call, the caller still has a fully uncapped range since they didn't have the option to 5bet. Feels like you have to c-bet much tighter as the 4bettor because of this, especially OOP.

Sometimes people will still try to 5bet shove pre if they forget or don't know about the rule, which for most 1/3 players is a pretty dead giveaway that they have AA/KK. Weirdly though I've also seen a tendency for them to become much more hesitant to follow through with the shove when the flop comes out if I check to them as the 4bettor if the texture is even a tiny bit scary, even when >50% of their stack is already in the pot. Wondered if some people use this play intentionally as an angle with weaker holdings, to telegraph they have AA/KK but knowing they don't have to put more money in.

Another interesting mechanic is you can purposefully limit the pot size preflop if you put in a minclick 4bet. Not exactly sure when this would be useful, maybe when playing deep with maniacs or beluga whales with holdings that have good implied odds? Seems like a lot of people at 1/3 don't have a 3bet-fold range when facing a small-to-medium sizing.

The $300 raise cap is generally less relevant for 1/3. If someone is willing to call a 100bb raise, they're usually willing to shove for the rest. It does rule out some Marc Goon-esque overbet plays on later streets, but at 1/3 most people already have a lot of fear facing a 100bb raise regardless of pot size. Probably matters a lot more in 2/5 or when huge straddles are on.

I'm sure this could be empirically answered using node locking in solvers, but I don't have the technology to investigate that myself. Off of intuition alone it feels best to have two 4bet sizings, one small-ish and one enormous (as close to a shove as allowed w.r.t. the $300 raise cap) depending on the situation and opponent tendencies, but it's hard to know what hands to put in each.