He's never been alone for 4 hours before? He's lucky.
I wouldn't worry too much about it. Book and Neelix both were confined to a single room while I worked an 8 hour day until they were 6 or 7 months old. Not together, Book is now 3 and Book is about 9 months.
Jasper should be fine. I'd just give him some toys and maybe leave some classical soothing music on. Chances are good once you leave he'll explore looking for you, maybe meow a little and then take a nap.
When I leave I always tell them I love them and if I'll be home before or after dark.