I think 4000rms is too much for (1) fully loaded 18" BTL.
I'm running a T300001bd that does about 3800rms with upgraded electrical system that consists of a 260 amp alt, double 1/0 gauge runs and (2) yellow tops. It pushes my BTL really really hard to about the point of full excursion.
If I run it at full tilt for about 5-7 minutes when playing a song by young jeezy I will start to smell the voicecoils heat up. I've only had this happen twice, but other then that it handles the power on normal rap music without out an issue. It's just songs by young jeezy and other hard hitting songs that really heat it up fast. IMO It wont be able to handle 4000 for too long before you smell the voicecoils heat up if you plan on playing it at full tilt for more then 10 minutes with a song that has a really killer bass line to it.
By the way, this weekend I clowned on a guy with (16) 10" P1 Rockford subs in a bagged navigator. It was at the pavilions car show and I heard him pounding his system so I pulled up to him and reversed into a spot right in front of him. I played some song by DJ UNK and everyone left his ride and walked over to mine. A few people in the crowd told me mine was much louder then his (16) 10" sub setup which I also thought too. His was more for show in small sealed enlosures anyways so it wasn't that big of a suprise that I was louder. He didn't even know how much cuft each sub was in. This was no suprise since most of the scottsdale kids here in AZ don't do there own work.