There's something we both missed. If this 4 year old is pumping out enough to "soak through his sheets", he shouldn't be letting it flow so freely while lying in his bed. Three possible solutions, #1. If he is about to go for a session of "tickling his penis", he should go to the bathroom and empty his bladder first. (May work or may not.) #2. If he can feel when the fluid is about to flow, as an older guy can when he masterbates, he should use the "squeeze technique", make a run for the bathroom, aim and let go. (Will work if he is aware enough to squeeze in time.) #3. He should "tickle his penis" in the bathroom, either in the shower or tub or on the toilet. (Foolproof.) If he is soaking his sheets every time he "tickles his penis", his parents must be getting impatient, and may think he needs to be back in diapers. He'll probably like any of the solutions I suggested better than wearing diapers.