i promise you the 18s is the better option. i had 17 cubes. for daily with 240in^2 i come out to 13.8 tuned to 36. it drops to 25 no sweat and hits hard as **** at tuning. if you did something similar you coulduse some deflection panels and put pvc and glass in all hte corners and go even more port area. it will hit hoarded than the 4 15s. no doubt. for your box you looking at 280in^2 with the same tune and 7.5 per driver. you could use 2 4kws and break that car without reaching the amps limits. you could even do 300in^2 and get 7 per..
the 4 15s might have a tad more output above 50 but they wont peak hard around tuning. for the 4 15s you need atleast 360in^2 in 15 cubes and even then that's close for comfort.