Quite frankly, given the amount of money you would spend on FOUR EP400s, you could buy THREE EP500s and still have money left over.
With the specs of the EP400 and that its innards are basically the same as the EP500, obviously the difference is going to be the size of the driver and the box.
I must admit, when it comes to subwoofers, even though the 400 is a wonderful piece of engineering, I have ALWAYS been a believer that, in the end, you cannot defy the laws of physics. The EP500 will go deeper and you will be able to drive it harder. Movies, especially with the newer lossless codecs are a real test of subwoofer performance at high volumes which is rarely the case with music. You can take that one step further by looking at the EP600 which has basically the same electronics as the 500, but once again an even larger cabinet hence specs which, "theoretically" will enable even deeper bass extension. As has been discussed many times, subwoofer placement will determine what you are hearing.
Unless you have a space issue, in my opinion, I would go with the bigger subs.
