I think the return chamber may be too small. This is the area that's got to be able to contain all the live water the system will need to be able to run. With only 4 inches, it can be very easy for the pump to run dry before the main tank overflow is returning enough water to to keep the sump's return area filled.
I'm also not a big fan of combo sump/refugiums, with the refugium in the middle. In my opinion this is at best a compromise design. It's limitations are that the refugium usually gets too much water flowing through it too quickly. Generally a refugium should get comparatively low flow, so that what ever is in the refugium gets time to process the water. Also, refugiums have a lot going on in them. With this design, it is not possible to cut the refugium out of the filtration system while you maintain it.
If possible I think you'd be better off making the refugium in an additional tank, and using a T and valves in the return pipe to supply the refugium. You'll get a lot better control. Best of all, you can treat this as another display tank. There is a lot of interesting stuff going on in a well done refugium.