I already installed four 4' 2 bulb fixtures (total of 8 fluorescent
bulbs) and the result leaves some room for improvement. It is just not
bright enough.

I suppose I could install, say, 2 more 8 foot fixtures for the total
of four 8 ft bulbs. There are some high efficiency ones, the last time
I checked, HD was out of them.


Try swapping for full-spectrum bulbs in the florescents, and/or
moving them farther apart.