I am leaving my boys for 10 days in a split 5 gallon. Your boy will be fine for 4, especially if he is already in good health. I have even more potential for things to go wrong. Not only is there a longer amount of time, but I have the lights on a timer because I have plants. If your heater has been working fine up until now, just as mine has been, don't touch it and go on your merry way. Who is to say that the heater couldn't fry your fish in the middle of the night? Sometimes you just have to believe in technology, even if it is scary.
Do a 25-50% change, but do it at least an hour before you leave to make sure that everything is perfect. :)