In that size box, assuming your pots would be about a foot tall, you'll probably want to veg to under a foot then flower. That 120w panel would do just fine in that space, you're working in an area about half the size of my NFT setup.

If you do wish to add light, I would suggest you obtain 4 of those single-strip Hydrofarm T5HO lights, where you can daisy-chain them from a single power cord and run them inline, and light up all four corners.