Your motors will run approx. 33% faster at full throttle which in itself shouldn't harm them unless maybe if they are really cheapos, and even then you probably won't be running a quad at full throttle anyway. Your throttle stick will be more sensitive so you may want to add expo and/or adjust the throttle curve in your Tx if it has those capabilities. If you fly at the same RPMs and with the same props as you did on 3S the overall efficiency may be affected somewhat, but it probably won't amount to much (but if you use a timer to determine when it's time to land, be conservative until you know how much of the 4S you're using per minute). Check motor temp by feel after a flight (a good habit to get into anyway). I've switched numerous helis and planes back and forth between 2S & 3S and between 3S & 4S and never had a problem (with appropriate BEC of course).
Thanks. About what I figured, but I am still new enough at this stuff that I like to ask first. Guess I'll give it a try. Be interesting to see how it all behaves.
What motors, props, and payload?