2001 FORD mustang amplifier count?

I have a 2001 ford mustang, and I'm changing the speaker system. New radio, new speakers (front and back). I do not plan on changing the subwoofer yet! I know the factory speakers are 8 ohms and my aftermarket speakers are 4 ohms, so I need an aftermarket amplifier. I'm just not sure if I need one for every speaker, one for the front then one for the back, or just one for all speakers. Please Help!

P.S. I'm not sure what amplifier I want so if you guys have any good recommendations please tell me knowing that I have a budget of $200 for just amplifiers, try to stick to the low side if you can.

I have a 2001 ford mustang, and I'm changing the speaker system. New radio, new speakers (front and back). I do not plan on changing the subwoofer yet! I know the factory speakers are 8 ohms and my aftermarket speakers are 4 ohms, so I need an aftermarket amplifier. I'm just not sure if I need one for every speaker, one for the front then one for the back, or just one for all speakers. Please Help!

P.S. I'm not sure what amplifier I want so if you guys have any good recommendations please tell me knowing that I have a budget of $200 for just amplifiers, try to stick to the low side if you can. If you've replaced the radio/CD head unit you don't really need an amplifier for them if you're on a budget. Most modern head units have 4 to 6 channels producing 15-23 watts each, one channel for each speaker. That's plenty for most door / shelf speakers unless you've bought some that are rated 100 watts RMS or more each (I repeat, that's RMS power per speaker). If you've bought speakers rated in the 35 to 75 watt RMS range, they'll perform just fine running off the head unit. If you had at least specified the brands and models of radio and speakers we could better help you decide what to do. (Fail).