My DS4.5 also has a late October birthday. Cut off here is Sept. 1st. We are going ahead and skipping him into K early. Actually we had him skipped into PreK-4 early last fall. There was no way he would have been able to do a 3's program at that point. He has been doing 3 hours/5 days per week this past year in PreK-4 and that has worked great. He fits well with the children although academically he is still way ahead.
Moving from 2 days up to a full day K should be an OK transition. We moved from 2 days (3 hours) to a full 5 days last year when DS was still 3 and he did fine. Might be a bit of a transition period for him but overall I would suggest you send him. Usually they do have rest time midday in K. At least at the private school we are sending DS.
Holding him back a year is a long time for an advanced kid to wait for more challenges. I feel you might run into the risk of severe boredom in the upcoming year. Skipping into K seems like a good option to me.
