Yes, it does make a lot of difference.

What power rating, depend on your speakers. But normally I would go for more,instead of less or ngam ngam enough.
But to satisfy general population, any REAL power between 50~75rms is very good liao.

If 4ch goes to front and rear speakers, then yes you'll need additional amp if you need to add a subwoofer.

BUT, if you already plan to add subwoofer, then just leave your rear speakers as it is, no need to route amp to power it. Because most likely you will not hear anything from the back....