I disagree with above and think it is
Calling pf with small pairs just puts us in reverse equity spots like this where our equity isn't great to begin with, then drops lower and lower as the hand goes on. It also makes it really easy to bluff against if villain knows these hands are in our range.
I understand you're grouping 99 with TT/JJ. Just out of feel I think 99 flops significantly less equity than TT/JJ on average and should be included in the small pairs category.
If you're flatting your middling pairs (not good enough to stack pf, not bad enough to fold), I think it should be with the intention of playing the hand pretty sticky. It also helps to have a good read on your opponent to know when/where to call down; I think if you've got a small sample and/or are readless, playing 4bet/call or folding pf is much better.
I think it's definitely a spot that's debated among better player than ourselves too, so I wouldn't lose sleep over it.
If you were on the button, and he 3-bet more like 7.5%, then you're in profitable 4-bet/call-shove territory
How so? %7.5 is 55/45 against us, and that's only his 3bet range, not his stronger 5bet range...I guess this means he has to fold a ton, like up to JJ and AQo, right? Not sure that happens often enough, but even so I'm pretty surprised it's that close down at %7.5 3bet.