Dad, just let him play with the 4 inch and play through it. You don't want to get it into his head that this has to feel just right and that has to be just right. That can manifest itself down the line in blaming the equipment, the weather, etc. That "feedback" at that young an age can come back in other forms when they get older. Let him struggle a little and tough it out.
Nadal's uncle used to make sure he had balls of varying bounces, odd racquets, and other hardships. In the end the ability to adjust and get through it will be more important than you shaving the grip down so its perfect.
Kids adapt fast. My 6.5 year old used a full size since age 5. She did great with ground strokes but I noticed she struggled when we went to lots of volley work. So we went down to a 26 inch frame. Sure the grip is a tad large and her hand gets tired after a while. She takes a little break and toughs it out. Her feedback is that she just wrings her hand out a little and goes back to having fun doing volley drills.
If your boy becomes a top notched 18, then you can cater more to his equipment. But at this point you want to teach him to adjust and adapt.
Sorry to bring up and old post, but I thought it would be better than creating a new one.
I don't agree with "Nick Bollettierri"
above. There's more than just the USMC or "Nadal" method of training a great future tennis player. I believe that proper, quality equipment (and technique) is the foundation for any and every beginner regardless of age. I've seen it in tennis, golf, guitar, etc. Good equipment makes a positive difference.
But I digress... My 5 1/2 year old son is 45" tall and is really enjoying tennis as well as advancing rather quickly. I want to move him up from a 21" to a 23" racquet, but currently his 21" racquet has a 3 5/8" grip (the smallest grip on the market in a 21" racquet). The smallest grip on a 23" racquet is 3 7/8".
When I look at his hand on the 3 5/8" grip of his 21" racquet, it is OBVIOUS that the grip is WAY too large. His fingers are practically 2 bevels away from the heel pad of his thumb.
Method #1... Not "gospel" but... Most adults are told "you should have one finger width between your fingers and heel pad". Why would this advice be any different for a child? Method #2... When I measure his hand (second crease of palm to tip of ring finger) I get 3" exactly... and I wouldn't say he has small hands at all.
So at a minimum, with his 21" racquet, he is roughly playing a grip size that is 1/2" - 5/8" too large. To put in in another light, that would be like me using a 5" - 5 1/8" grip instead of my 4 1/2" grip. So to move him to a 23" racquet with a minimum sized 3 7/8" grip makes no sense at all.
So my question is this: Why would EVERY racquet manufacturer neglect such a rudimentary and critical factor?
I'm not suggesting that the manufacturers start making 21" - 25" racquets each with multiple grip sizes (obviously the economics wouldn't pan out), but I am suggesting that they do a better job researching what sized hands children actually have. Wouldn't be that difficult and it seems quite obvious, basic, and important. After all, children are potential future customers and what better way to get them hooked than by providing them with proper, quality equipment.