One agitation cycle per minute is very normal. The regular numbers are one cycle per 60 seconds or one cycle per 30 seconds. Note that "4 agitations" in itself doesn't say much, but in practice, it doesn't matter much either as long as the motion succeeds in achieving turbulent flow across the entire film surface. This will be the case with general inversions as shown in many youtube videos, described on websites etc.
When agitating, keep in mind that the whole point is to actually agitate, so don't be too gentle. Some people do inversions as if they're balancing an egg on top of the tank; there's no reason to be that cautious and all the more reason to be a bit more brisk.