Planning on doing a wall? Planning on taking out the back seat? Planning on still probably needing more room for the subs to work well? If it's SPL that you're looking for, I would suggest 2 subs properly enclosed over 4 that are going to want more air to work in. Also the subs, amps and required upgrades to the car's electrical system to support it all will cost a lot less. You aren't going to do 4 15s on a tight budget.
Get 1 18" MT by Resonant Engineering. It's going to be cheaper than any set of 4 15's and you can put the extra money into some nice amps and a quality install. If you have room for 4 15" subs you should be able to give that 18" a great box.