Well the 400w light is a little small for a 4x4 grow so after this one you may want to ad a second light or go larger with one light.
Second you are better off with a 6” net pot or better yet an 8” net pot. Just make sure that the light does not get to the roots or to the bottom of your grow tray. Black plastic strips of pond liner will cover the area fine just remember air flow is also needed so an air stone or two will help that when you have it covered.
Your mix is ok but there is not much there for the roots to grab hold of so add some grow rocks or lava rock to give the base some girth so the plant will not fall over. Even if you are filling the whole 4x4 with your grow medium the roots will need something to grap a hold of or the plants will tip over.
Good air flow and temperature control is a must for a good grow. Change out your water and nutrients every 7-10 day’s and make sure you use good quality water like RO water if you can. If you must use tap water then make sure you let it set for 24-48 hours before you ph it and add your nutrients.
A light mover may help spread your light around a little more or the plants on the out side of the table will all grow toward the light and interfere with the other plants getting quality light as well.
I know of many who have grow up to 30 plants in a sea of green in that same area but they did use 1000w of light too.
Watch the temperature at the top of the plant canopy under the lights.
If you go cheap you grow cheap. - Me