Be carefull when replacing your front speakers. They are wired in parrallel. The 4' ones in the dash are 8ohm (I pulled mine last week) and It's likely the doors are 8ohm too. They are 8ohm so the head unit sees a 4 ohm load.

If you replace the speakers with standard 4ohm aftermarket speakers, the head unit will see only 2 ohms, which can cause the tiny amp in the head unit to overheat due to greater current draw.

You can take a chance that your head unit will be fine (2 ohm stable) or you can do away with the 4" speakers and go with just s good set in the doors. One other option is to use a 4channel amp to power all four.