For the price and effort of framing w drywall you can just get one of those grow tents. It’ll be lightproof etc and have all the holes for your ventilation and a door. Unless you need some weird custom shape they’re ideal.
4x4 is a bit large for a 400w light, but personally I like to have extra space. You can put plants up elevated around the sides to use the extra room.
It’s hard to say for veg, will you be vegging the next round of plants while the others are blooming under the 400, or getting them to size in the 400w room. If option a you’ll need enough room for those plants + a couple mother’s/clones or whatever.
Personally I’d go with a 3x3 or thereabouts. Get LEDs for your veg space they’re much more manageable than HIDs and use alot less power.