Any suggestions? I was thinking about 4 8" subs or 2 12" but I'm not sure which to get or who to go with. I've been getting mixed so any help would be appreciated. It's a 2002 xlt, previous owner had subs so installation should be easy.

 

The larger the diameter of the speaker the lower the frequency it can generate, so if you can fit it in the 12" or 10" minimum
And its not just the speaker size its the enclosure its in, "the box"
Google: building a subwoofer box
Just so you know why "the box" is important

The other thing is a crossover frequency
You need to know the low frequency of the mid-range speakers you have, if they are rated for 90Hz then you don't want the sub to output 100Hz or you will have "wave cancellation"
Two like frequency waves will cancel each other out
So the 90-100Hz will sound "muddy"
You set the subs highest frequency BELOW the mids low frequency

Powered subs are better, and use the large Capacitors on the 12volt power wires to these, easier on the alternator and prevents "dimming lights" on deep bass notes, lol

 

I've got 2 12s in my Ranger and to be honest, it's a bit much. But in my younger days, I wanted to stupid loud with it 😁. In my 7.3, I've got 1 12 behind the seat and it suffices. Also doesn't take up the whole back seat like the 2 12s did in my Ranger.

Brand wise, it's all up to you. These days it's kinda get what you can as opposed to what you want. I have pioneer 12s in the Ranger, but I went with Skar in the f350 and I've been much happier with the Skar.