ok my head unit is 50w x 4 so can i buy 4 50 watt speakers and replace the old ones or do i need an amp. or could i just switch the old speakers out and replace them with the 4 new ones.
Most likely its putting out 25w RMS(50w Peak). I'd reccomend just buying a amp for the speakers, sounds better overall if you have decent-good speakers to run it on.
but now with speakers when i hook them up to an amp to i have to match the rms to the head unit or can i go higher now that i have a amp or do i still get 4 25rms speakers just hook them up to the amp?
You can go higher with the amp, It doesnt matter what the RMS of the headunit it cause your not gonna be useing it. So find a amp, And get speakers that are in its RMS range