If you are looking for decent audio in your truck you should at least install 6" x 9" in the back, did it myself, very easy just cut off plastic where needed and secure them with long screws to reach the metal plate to minimize vibration when speakers are pounding hard, in the dashboard you are stuck with 4".
Consider the Pioneer TS-A6995S, these are 600 W Max (100 RMS) 5 way 6 x 9", installing an amp is a must to get the most of the speakers and believe me they do deliver, they are a very good bargain at around 70 $ or less.
I am facing a small problem now, even though the sound is nice and the base is awesome, the speakers are faced to each other and not pointing to the front and this is bad, i'm thinking of a way to re-direct sound waves to the front and i will share it here if i get a good result, and by the way if someone already figured out a solution for this sharing it with me would be great.