My son just started Pre-K in The Einstein Academy in Elgin (right of I 90 & Rt 31) and he absolutely loves it! They go one grade above public school curriculum (so now they are going over Kindergarten books). They have many fun and interesting things that they are doing every day (swimming, art, Latin...). However, the school year just started, so you might need to wait until the next year to enroll him there.
When it comes to Karate, my son took it in the summer (he was 4 in March, so is little bit older than your son) and he did pretty good. It is very personal preference, because the teacher did not want parents in the classroom while the class was in session and the kids that disturbed the class had to do push ups. But I would still recommend it.
Let me know if you'd like any more detail on any of this.
Thanks!
M.