The 4 way uses 1 1/2" outlets and and 2" inlet. The Hammerhead can use either 2" or 1 1/2". I don't think the pump will be too much flow on a 400. My tank measures 7' L X 2.5' W X 3' T. I was using 2 hammerheads, one for a return as stated eariler and one on a CL with the canister filter.
I'm now going to use a dart on my return, a hammerhead on the CL with the filter and a barracuda on a CL with the 4 way. My sump return is 2 X 1 1/2" over the back w/ 45 degree to point the water towards the corners. The CL hammerhead intakes thru 4 X 1" ports in the center overflow and returns thru 2 X 1 1/2" up thru the bottom. The other CL will feed thru 2 X 1 1/2" intakes on either side of teh center overflow and return thru the 4 way, one 1 1/2" port in the upper corner on the two ends. I'll be using a drum that only opens one port at a time so I can get a back & forth wave action.