it may depend on how 'serious' you want to get with the setup. IMO, you should run all speakers off an outboard amp for best performance. as such, if you really want rearfill, run them off an amp.
an important reason is that an outboard amp will have suitable inbuilt xovers to HP your rears (and front splits) for best performance. otherwise, if they're run off the HU with a fullrange signal, they'll distort prematurely (way before the remainder of your system does) and be very annoying...and you'll probably disconnect them as a result. :wink:
don't place too much emphasis on having excessive power to run splits and rears. unlike subs, splits are very easy for any amp to run (cos of their small size). the hertz HSK splits (in your sig) will go uncomfortably loud with 85WRMS on tap. that's way more than you'll need for normal daily listening.
you will NEVER use 150WRMS per channel to run them, or else they'll fry. sure, you can have a more powerful amp, but the way that you'll need to set the gains...you'll effectively be capping how much power the amp will ever produce, even with the volume up full (eg: capped to 100WRMS or whatever). which IMO, is probably a waste of money in this situation.
however, if you can add a 3rd small amp for rears (even later), it may be worth going with a solid 2-channel amp for now. it would also cater for any upgrade of your splits in the future (again, be realisitc and determine how 'serious' you are about the stereo).
so if this is likely the last amp purchase you'll do for a long time (ie: years) AND you want to run rears, then i'd go with the 4-channel.
but if you're really into SQ, and are still building up a bigger system, go with a nice 2-channel now, and add other stuff later.
good luck