I have both the xp3 and xp4... and they seem to be the same filter, except the xp4 has 4 baskets instead of 3. Best value, brand name canister filter, I say the xp3. Both are Rock solid, super quiet, and easy as heck to set-up and maintain. The best thing to do for this filter is to put a sponge filter on the filter inlet. I used to lose some flow every 2 weeks before I did this... but I have Java Moss which loves getting in everything. Each basket in both the xp3 and xp4 have 2 chambers...in the xp4 I filled the 1st basket with the 4 supplied micron particulate screen/sponges, 2nd basket with 4 layers of blue pond filter mat, 3rd basket with ceramic noodles, and the 4th basket with the supplied bio-stars in the lower chamber and the supplied carbon bag in the upper chamber topped with a floss screen. Great water polisher! I have this xp4 in use on my 150g with a over-tank sump as well. My water is crystal and I overfeed a large pair of Oscars and a dinosaur sized Pleco. I would most definitely buy another Rena! For how good they are, you just cant beat the price and just how super quiet they are!