I've found over the years that having light at a low angle helps me to see overspray,runs,etc. when I lay down the paint.So I put lights at about 6-7 ft up on the walls,as well as the overhead lights. If you look at the professional spray booths,you'll see they do the same.If you cant do that,then the next best thing is using the portable lamps and move them across from where your painting.
In a spray booth, because the size and proportions of the booth are close to that of a vehicle, most of the over head light is blocked from reaching the sides, by that vehicle. So, lights in the side walls are preferred. They are located low on the side walls, around 2-3 feet, because the distance from the sides of the vehicle to the walls, is usually barely 4 feet, and an automotive booth is usually under 9 feet high on the inside.
But, not all spray booths come with them. Having used a particular Canadian booth, which did not come with side lights, I can attest to the difficulty this system often creates.
If your just doing pieces, and parts, as he will be doing, (he can't get a car in there), plenty of light will reach the side walls, which will reflect on to the parts, so this will not be a problem. And, it never was a problem doing pieces and parts, in the booth with ceiling only lights.
There is no such thing as too much light in a spray booth. But, a lot of strong light can create glare, which will produce unintended difficulties. If a bright light is behind the object your trying to see, then it can become more difficult to see it well.
I don't think he will need side lights in this case, with 4 giant CFL's he will have 1200 watts of light. That's a lot of light bouncing around in a small space. I have 10 of them in a 30x30 shop, and it's very well lit.
