Is the main purpose of this knife for skinning? If it is, the 4 is the clear winner. If it is going to be a general use knife, maybe not.
The 4 is still my favorite because it is so universal. The 5 however is more capable of harder uses and abuse due to its ¼ thickness. It also chops pretty well because of the weight it carries.
I know how it is to want something as a kid and insist and insist, with Dad knowing all along that I would be better suited with something else. I can think of 3 times in my childhood I wanted a particular knife and tried to talk me out of it. 2 of the 3 are in the garbage somewhere and the last one I sold on the exchange.
Id give him the 4 for Christmas and tell him you got a good deal on it and to see if he liked it. After that he may decide the 4 is the perfect knife for him, or it might start his ESEE addiction and hell start mowing lawns and collecting cans.
ETA: If he is going to be carrying it a lot, he'll be glad he has the lighter weight of the 4 I think.