I use CMH and like it. You're right that you'll need to fill in the edges in a 4x4 tent (315w CMH is for a 3x3 tent at 35w/sq ft, not counting the ballast). I don't think you're 200w MH will have a good footprint for this.
If you think of it in w/sq ft, you have an additional 7 sq ft to provide for. I.e., if the CMH were sized for a 4x4 tent it would be a 560w). So, you need 245w additional.
How much can you spend on supplementing that additional 7sq ft? How much sweat equity do you want to put into it?
You could use five 4-foot (54w) T5HO flourescent tubes which would give you 270w. Those fit nicely between the tent legs as downward sidelighting (when the plants grow into that space. Otherwise, leave them turned off. You could do Agroflex two-tube fixtures. Or, SunSystems has a "premium" 1-tube with decent reflector. Those would be relatively easy to hang and manage.
If your on a budget, you can use ordinary household LED lightbulbs. Attach them to the tent legs. (You can make a "tent leg" between the 4' tent legs using PVC pipe, and mount lightbulbs from that for better coverage.). Before you think that's hokey, I've grown
a plant through flower (<<link) and it turned out pretty well at just 18-22w/sq ft. I really like the lightbulbs. I think they're underutilized by growers. But, they can take some work getting setup, an managing them in a larger space.
As an example, you can mount these $3 PAR38 floodlamp holders in a 1x4 board:
Use some 19w 40-degree floodlights. You need about 30w/sq ft of this kind of lighting. That would be 12 bulbs. (Three for each side of the tent.).
There's quite a few ways to do this. I would use the floodlights that you can cut the front lens off, and have the surface-mounted LEDs facing the plants (directional) without being concentrated into a 40-degree beam. Less absorption loss. (You can do the same thing with 8.5w 60w-equiv "lightbulbs", but you'll have to mount 6-7 to each 1x4 board. Those bulbs only cost $1.25 each. More coverage having the watts spread out more.).
If you're interested in this, it's not hard to do. It's just DIY stuff. All you need is at the hardware store. The mounting part can be an initial investment of sweat equity. But, after that it's relatively cheap to replace bulbs (lumen deprecation, every 2-3 grows). There's a number of ways to do this. You can mount them to the tent legs. Adjustable up/down.
