Have you checked the dimensions of the cooltube fixture? Is the cooltube with ducting on it to vent the air going to be able to fit in there? By the way. I grow LR2's in a homebox s (which is 31.5 X 31.5 X 63), with a 250 watt hps and that works well for me. I cool it with a 6 inch inline duct fan, and that works fine for that. You may already know this, but I'll say it just in case. A 250 watt covers 3X3, and a 400 watt covers 4X4. With the 250 watter covering 9 square feet and you only having 4, I think it's plenty of light for that. Plus since you're growing smaller plants, you don't really need a more powerful light to penetrate a deeper canopy. Now if you are going to go bigger later, you'll outgrow (figuratively and physically speaking) this setup very fast. But, if you're ok with this setup and this strain or any of the other LR strains then I think this setup is perfect.