I have the new 4000GPH pump and its awesome! The pump itself needs modifying just a bit. As for your pool, get rid of the plunger valves. They restrict the flow by about 1000GPH or more. There is a plastic grate in the inlet side of the pump. I just took a small file and sharpened the leading and trailing edges to smooth things up. The biggest thing is with the plunger valves. I modified some PVC fittings so I can use 1.5 inch ball valves. When my pump starts up, you see the water moving. I no longer have problems with **** settling to the bottom of my pool. Oh, and it will run an automatic cleaner with ease.
I dont recommend using the 400GPH pump with hoses smaller than 1.5 inches as there will be too much pressure and is likely to cause problems.

My 18' round intex pool pump wasn't working well after 3 yrs, so I bought the 4000 gph pump. When I first start it, it takes off like a rocket and water comes out strong, but shortly after barely any water comes out. Any clue whats wrong?

How do you shut your water flow off to change the filter?