Hey,

What is a good F-stop to keep your lighting at? A pro grip I believd told me he keeps it at around 4. Is that correct? Any feedback appreciated.
Duke

The iris/f-stop is one factor that determines the depth of field in the picture.

If you want a shallow depth of field (defocuses the background; it may look good, or not) then try to keep the iris open / keep the f-stop low.
Shallow DOF makes maintaining focus more difficult. For studio productions deep DOF may be nice... i.e. The Price is Right (game show)
Sometimes deep DOF is desireable... i.e. Citizen Kane, lets you see everything in the background.

In terms of image sharpness, the lens will be sharpest away from the extremes.

There really are no rules for this. Lens generally provide a "better" image when away from widest and narrowest aperture. It depends on the look and effect you're going for.

It's a bit of a stretch for a grip to announce that he is maintaining a stop; the DP and the gaffer are the two folks responsible for metering (and the DP passing on the exposure to the camera department). The grips will assist in maintaining the stop via nets, bounces etc. as required by the DP.