OK, your wet/dry is totally wet.. there sill be no air on your pot scrubbers, so you loose a whole bunch of bio filtrations there.
stage 4 at least has air being pumped in to help, but I think making a drip plate over it & lowering the water line will be better..
My thoughts.. start with a filter bag in area 1 & 2, over flow into stage 3 but move the drip plate to area 4 overflow from 3 to 4 via the drip plate & raise the wall between 4 & 5 to allow water under it allowing the bio area to be a true wet/dry with a lower water level..
your current config will require cleaning of the pt scrubbers in area 2 often
I can make a sketch if you like
stage 4 at least has air being pumped in to help, but I think making a drip plate over it & lowering the water line will be better..
My thoughts.. start with a filter bag in area 1 & 2, over flow into stage 3 but move the drip plate to area 4 overflow from 3 to 4 via the drip plate & raise the wall between 4 & 5 to allow water under it allowing the bio area to be a true wet/dry with a lower water level..
your current config will require cleaning of the pt scrubbers in area 2 often
I can make a sketch if you like

