gator,
Hey there.... thats a cool design though you might want to consider a 6" PVC instead of a 4" diameter.... i have 1 DIY PVC canister filter in my 50gal.. its my back up filter and its mainly for bio filtration... my main filter is a 3ft glass ohf with 3 compartments.... the PVC canister conatains alot of pot scrubbers, foam, 1 kilo ceramic ring and just 1 layer of filter floss.... just for polishing.... i dont want to put too much filter floss in my PVC canister because if i will put to much the maintenance will double up... i dont want to worry about the flow rate of my rain bar just to check if my floss in my canister is starting to clog up.... the pump is located at the top of the canister and the inlet is located at the bottom... i choose this type of set up in a way so that it is easier to service the pump if ever it gets busted, the water will be gravity fed so the pump will not work double time also because placing it above the PVC pipe will reduce the head height of the pump... so say your pump can pump only upto 3 ft and if your tank has a height of 3ft you will loose alot of flow rate but if you put the pump at the top of the pvc you get almost 0ft of head height inturn your pump runs in its full capacity....
btw heres a link of my build... its in our local forums here in the Philippines so some words are written in our vernocular..
http://mypalhs.com/forums/showthread.php?t=88596
heres a suggestion.... why not build your DIY PVC canister and instead of using a 4" PVC use a 6" PVC... try to compute the volume of the 2 with the same height... you will have a bigger volume capacity... also to lessen the plumbing work why not just series the whole canister??? 1->2->3->4->pump instead. heres a drawing... sorry if its not that clear....
notice the color red in the drawing??? this will be the lid where you can do your maintenance work.... much more simplier and less plumbing... coz in this type of DIY we dont want to have air/water leaks it will defeat the purpose of a pressurized canister... so less parts less hassel...
but if you can get a hold of a 6" PVC you can just use two i guess.... then about your sump i suggest you keep it.... if your having problems with your water level you can add an elbow on the current bulkhead in your tank then angle the elbow approx 45deg that way you can increase your water level... its much better if you have a back up filtration...
just my 2cents....
your friend from the south east....
DJ