RES Tampa--you were joking?
If not, I'll just say they'e two totally different types of filters. The Fluval 4+ is an internal filter, which takes up room in the tank (this is not the same as the Fluval 404, which is a canister filter like the XP3). The XP3 is much more powerful and efficient; I've never used the canister Fluval (404), but like I said, it is popular with some turtlekeepers. I'm using the 4+ in a 40-gallon tank; I'm using the XP3 in a 110-gallon tank.
That said, I'm looking for a filter to replace the XP3. Most likely it will be a pond filter, since it has to be on the floor next to a stock tank, and canister filters don't work well this way (as the XP3 keeps telling me with the noises it's making). Canister filters work on gravity and have to be a certain distance below the water level to work really well.