I also have a 4-door Wrangler and you will need some type of rear fill. The cabin is just too big and if you have passengers who you want to enjoy music as well you're gonna need rears otherwise they'll be in a "void" between the highs & midrange from the front and the bass (assuming your sub is in the cargo area). Remember, Wranglers are horrible for SQ as 3.5spd said...road noise and just bad acoustics overall.
Did you install the fronts yet? The dash speakers take some doing to remove. The driver's side is the biggest b!tch to get out.
Here's my Jeep's setup.