I don't think that the fact is that fleas cannot live 4000 feet above sea level. There are plenty of fleas in the mountains here, and these mountains are well above 4000 feet. Perhaps where your daughter lives, they do not have flea problems. I also do not have the need to control fleas where I am, but I choose to do so anyway, along with heartworm medication, *just in case*!
Not being a bug expert, I do not know if there are altitude levels at which fleas do not exist, but I am sure if you ask terminix or some other bug control company, they can inform you if there are such altitude control for fleas. :)