Update: no go. (as I expected) The teacher said it was not only the material, but because there was going to be group work/interaction that she didn't think a 4 year old could keep up with.
Socially, though, at this age the older kids may well shut your child out on grounds of size/appearance/voice/apparent maturity, regardless of intellectual capabilities. You can't assume they'll be generous and inclusive, and that's also hard to force. We have had mixed experiences in this regard. The girls tend to baby-talk (our physically small) DS and he hates it. Intellect is simply not the whole story with activities like this.
We do choose multi-age camps and activities where DS is on the younger end of their allowable age range...
