You should try getting your plants to a hight of at least 45-60 cm imho. also, you should top and LST the shit out of them (not the auto!).
definitely throw in more lights! 400w can be used in that tent size, but it is on the low side and you have WAY to many plants in there for a 400w to cover that shit!
A quality 600w bulb can give you almost twice the amount of lumens for only 200w more.
Another good option would be if you get another 400w ballast and go 800w mh/hps combo.
You will not regret it! ofc its
gonna cost a bit extra BUT if you use IE a 600w for bloom 12/12 it will consume the same amount of energy as a 400w in veg 18/6.
400w X 18 hours = 7,2 kwh
600w X 12 hours = 7,2 kwh
800w X 12 hours = 9,6 kwh
Ofc you could always put in some 25w cfls to get a bigger lightprint. thats probably the cheapest way to go. but with HID light you get better yield and quality.