Honestly, I upgraded my door speakers (Alpine SPR-17S) and added an amp (Alpine MRP-F250) and it fills my car quite nicely. Granted you need to have a decent equalizer on the head unit to get the most out of it. I had the 4800MP you have right now and that equalizer is absolutely terrible and confusing as hell.

Since I've changed to a Pioneer Z1 I've gotten more out of my speakers and amp due to better equalizer customization. I might not have to get subs anymore because I still need to get my rear subs/mids rewired into my system. Right now I only have the door speakers wired and the sound is great.