I'm going to try to answer my own question just from my own observations watching hand replays from deep stacked high stakes heads up games.
From what I saw, when deep stacked, the 3-bet/4-bet/5-bet game actually turned into something more like the 3-bet/4-bet/…/n-bet game, where the n-1 bet is the one that is pot-committing, and the nth bet is a shove. Thus, rather than making a small 4-bet, you'd make a small n-1 bet.
I saw this in a hand with Isildur1 against durrrr (I think), where they were both sitting on more than 250 BB. durrrr raised to 3 BB, Isildur1 3-bet to 9 BB, durrrr 4-bet to 27 BB (full-sized), and Isildur1 then 5-bet small to something like 60 BB.
At this point, the SPR is about the same as a medium-stacked 4-bet hand, and post flop I imagine it would play similarly.
My guess is the main difference is that when deep stacked, you can widen your range for 3-bets and 4-bets, since the stacks are deep enough to still allow for post flop play. However, each progressive raise further polarizes each player's range to the point where the small n-1 bet is probably KK+ or a bluff when very deep stacked.