I got two plants under my 400W w/a scrog screen in a 2x3 footprint. I may do 3 or 4 in the future, with less veg time.
Eight plants may be too many. You could do it they were small, or if you were OK with less light coverage. I read where a 400W projects an optimum footprint of 2.5 x 2.5'. I also like 1 sq ft/plant. Put the two together & you are at ~4.
Your space is 4x4. You could do 8 with the 400W, even better if you supplement your light with CFLs/T5. They look OK in your photos, not too crowded (yet!). Keep going & see how it works. You will learn & adjust as needed for next grow !
I am running two plants now to sort out my new grow box. I may do more in the future, especially if I want to grow a couple of different strains. I think I am better at fewer plants trying to grow well & maybe higher yield, rather than running a bunch of smaller plants.
A quart of water every of day may be a bit low. Watch your plants - you know best being there. But I like to get some runoff when I water, and a quart for 3-gal pots sounds a little low. Try & do a search on 'how much should I water' & see what you get.