Actually - some Pentium MMX are locked. I have 166MMX and I couldn't set multiplier higher than 2.5x. And another 166MMX ran happily at 3.5x.

Anyway, regarding PPro - 3.5x is definitely possible, I used that myself many times on several boards. 4x is also possible but most CPUs won't run at 266 MHz, some might work at 4x60 = 240, but it it really faster than 233?

PPros with 1M cache aren't ideally flat with a kind of dent towards the mid of the cooling surface. This can lead to poor contact of the heatsink in the middle of the CPU if a cooler from a "regular" PPro with 256/512k is used (which have their "highest" point in the mid of the CPU). There are coolers with somewhat reduced contact surface with only a narrow strip in the longer dimension making contact. I think those should be better suited.

I remember even Intels datasheet for the 1M model having some information about that issue but I'd have to dig around right now to find the exact location.
Potentially this could lead to some virtually bad cooling results with higher temps.