I have a tiny 4 yr old and everytime she went to get in line, they would ask me or her how old she was. Once I said 4, they always let her on, she was above the minimum height. But I did see a very little girl go on the slide very often, I would say 2 or 3 at the most. I don't know how or why they let her on. She would hold up the line every time and the mother would have to coax her to go down.
What I do, it will be easier if there is two adults, but when mine went down the slide, it was just me. I would walk them up to the start of the spiral steps that was at the very top. I would wait there and let them get in line. Once they got about 5 or so from the front, they lifegaurd would usually start scanning the line and ask, I would tell her how old she was and then get myself down to the bottom of the slide for when they came down as fast as I could. I was exhausted by about the 3rd trip down.
If he goes alot at the same time, after about the 2nd time or so, they should recognize him. I would stand there incase they asked, you could say 4 or if he said 3, tell them he is still used to saying 3 since he just turned 4. That way you are not teaching him to lie if you coax him to say he is 4. Also, some will let him up, so if you want to take the chance, you can tell the truth. Most will proably let him up, some won't. Also, if he is big for his age, they might never ask.
Yes, it is a slide off to the side of the actual pool but there is tons of water and the mat stops them at the bottom. They do not end up in the actual pool.