4 channel amp - run 4 channels or two front and a sub

So idk where else I’d ask this.. I have very limited space in my vehicle and have been deciding on how to get the most out of my space/equipment/money. I bought a 4 channel amp and two new front speakers.

Since there’s no factory rear speakers, I’m torn between adding in the two rear channels or bridging the amp output to run a single sub.

Maybe eventually I’ll do both but for the time being, what would you do? I know the sub would bring back the lows that I need but I also think that two rear fill speakers would help considerably as well. Curious on your thoughts!