assuming that past impeller issues have been fixed, the main problem with the XP4 vs the XP3 is that for all the extra money you pay for the XP4, you only get small increase in media space. they just added a top basket, but kept the base size of the canister the same.
so instead of making the largest model taller and wider, for a big increase in media volume, they just made it slightly taller, so you don't really get much more for your money.
the extra flow rate is nice, but the XP3's already have pretty good flow.
I love my XP3's though
