5 channel amp would be ideal, but those are rare.
A 4 channel and a single sub amp also works.
However, the placement of the rear speakers in the Z3 mean you won't hear the rear speakers anyways, so why not just go without for a while and see if you miss them.
Then you can run a single 4 channel and bridge the rear channels for the sub.
They are pathetically small speakers and aimed at the rear of your seat.
2001 Alpine / Schwartz Coupe
2001 Estoril / Estoril Coupe
2001 Steel Grey / Estoril Coupe
1999 Estoril / Estoril Coupe
Sunroofs are for girls.