I wouldn't go with F18s. Choices are not good and really not worth the cost. The same $ will purchase better quality F32 bulbs.
Overlap 48 inch T8s and over drive them as you planned. If the middle of the tank should be too bright, paint a foot or so of one or two tubes to block it. I doubt 4 over driven T8s will be too bright though. I've had four on my 48" 70 gallon and thought that was quite nice. You might want to leave some room for another pair or two pair in the future.
BUT...
If you have to purchase ballasts, consider starting with a two tube T5 HO setup and adding more later. The next addition would be reflectors that clip on each bulb. Then possibly more tubes later.
Overlap 48 inch T8s and over drive them as you planned. If the middle of the tank should be too bright, paint a foot or so of one or two tubes to block it. I doubt 4 over driven T8s will be too bright though. I've had four on my 48" 70 gallon and thought that was quite nice. You might want to leave some room for another pair or two pair in the future.
BUT...
If you have to purchase ballasts, consider starting with a two tube T5 HO setup and adding more later. The next addition would be reflectors that clip on each bulb. Then possibly more tubes later.