Since Settlers is ported to x86 there should not be a problem to port it to ppc as well. They must have codet it pretty machine independent. I have not seen Settlers 2-4 on Amiga so they might have changed the codebase to something more specific to x86 after version 1. The only thing I know is that my mother would not play any other game that Settlers.
Well, she played Impossible mission on C64 and Impossible mission 2 and tetris on my A500 almost 10 yrs ago. But otherwise, only settlers after that (1 & 3 is still boxed and in my mothers possesion). After she got tired of Settlers 3 she stopped playing computer games. I have tried with other rts games, but no go.