i think you mean undescended testicles (ones that are not in the scrotum where they belong but rather in the abdomen). at age four bringing them down now is advisable. it would be highly highly unlikely that future fertility would be adversely impacted since you are doing surgery at this age. i think you will end up with plenty of grandchildren.
my 4 year old was diagnosed with descending testicals. has this already affected his fertility? . my son is 4 and was just diagnosed with descending testicals. what are the chances that he will still be able to have children of his own when he is older if he has surgery to correct his condition now.