Hi M. S,
I can tell in your posting(s) how upset you are. Your heart is obviously aching for your son because you believe he is lonely. Just because he is an only child, doesn't mean he has a hard time making friends, because you have said he DOES have friends.
Often times, when kids are "OK" students, or struggling students, they find their "security and strength" in their extra-cirricular activity. This can sometimes give them the confidence to be a better student, or even be a more social individual.
Physical sports are not all we can sign our kids up to do. There are music lessons, art, drama, etc... It sounds like you believe that your son needs a "creative outlet". It couldn't hurt....as long as he doesn't struggle to get homework done without it.
I think your son is shy, and he feels like he doesn't "fit in". That is not a good thing in 4th grade, because as he "develops" these next 2-3 years he will struggle to become more secure with himself due to the changes in his body and voice, etc....
And, did you say that YOU go to school early with him to play 4-square? I thought so......THAT is your problem :O) You have been his best friend for SOOOOOOOOOO long, and you have LOVED that title (as any mom would) BUT you are invading the time he is SUPPOSED to be trying to make new friends. How many years have you been doing this? This is why he is socially shy, because his mom always "steps up" to make sure he always has a friend. INSTEAD, drop him off and let him be......he will figure it out, like all kids do.
M. S, please don't go to school to play on the playground with him anymore. You don't want him to have to ask you "not to come". He is in 4th grade, and needs to figure this whole thing out before he reaches the more socially difficult years of school.
Now, if your son has a learning disability, or other handicap that you didn't mention.....then I'm sorry for being "hard on you" about being on the playground. I can understand being there if you son has those issues.
RELAX and let him enjoy the life he already has.....without his mom on the playground :O) "LET GO..." he'll be OK :o)
~N. :O)