Sounds like you understand that 400 PPM is a problem now that you are using GH. I used to use 4" cubes for years (you know that) and I hate them. I'd much rather use 3x3x4" cubes instead and put them in 8" pots of hydroton. I also did that for years and it was pretty great but adds slightly more work because you have to transplant into pots and wash hydroton but your plants will be much better.
I have ChemDD plants right now in 2x4' Ebb and Flow Trays in VEG. There are 15 plants per tray in 5" net pots which are very small. They barely fit any hydroton and they are getting over 1.5 EC / 750 TDS and are mad healthy and grow real fast. The point is that I am using much smaller lights and they can still handle 750 TDS and I flood every hour which they really love. Hopefully you figure out what your problems are.
I have ChemDD plants right now in 2x4' Ebb and Flow Trays in VEG. There are 15 plants per tray in 5" net pots which are very small. They barely fit any hydroton and they are getting over 1.5 EC / 750 TDS and are mad healthy and grow real fast. The point is that I am using much smaller lights and they can still handle 750 TDS and I flood every hour which they really love. Hopefully you figure out what your problems are.