'inversions'
Yes, it is good to agitate (the usual english term) film and or paper in both the stop bath and the fix. Agitation in film developer needs to be at regular intervals - with some developers more or less than with others - but four inversions per minute sounds like too little to me. Read the directions for the particular developer you are using, they should give you the proper amount of agitation for that developer. With the fixer you should agitate for, perhaps, one minute, then look at the film to see if it has cleared. If not, continue for twice the time it takes for the film to clear.