Easiest way to check if it can handle 4S is to get a watt meter. The motor doesn't actually care very much what voltage it runs on, it's amps that kills it, but a higher voltage will make it draw more current as well. Measure the amps at 3S. If you are under the rated 27 amps you should be good to go with a 4S, and try with that. Keep in mind that it will draw less current in flight, than when testing static thrust. You could try to actually fly with a watt meter, to get an accurate reading of the max amps.
