I don't flush plants these days, but I've tried it on a few crops. I just can't tell the smoking difference between those I flushed and those I didn't.

Plants store about 1-2 wks worth of nutes. I'd be hesitant to stop nutes 4 weeks before harvest. I think that if I stopped nutes in wk 4, plants would be looking fairly deficient by wk 8 (or more likely, by wk6). I'd expect an associated yield reduction.

If you want to flush, a max of 2 weeks is probably closer to the mark.