i586 or i386
There is difference between i386 and i486 and i486 and P5. And it's not only regarding "backword" support. i486 has some features that are not supported by P5+. Also about backword compartability of IA32 - it's partial any way. For example (16)20bit real address mode rounding (when segment*16 + offset >= 2^20 in modulo 2^20 addition for 20 bit address bus) is OK for i386-,
yet causes PF in i386+, also there are difference in NMI handling etc. So for carifull system programmer I'd say - there is only partion backward support.