K.D.
Wow, it sounds like this is going to be hard for you. As far as sep. anxiety in a 4mo, yes, it is possible. It's not common, but it does happen. It's really great that your family can watch him, and also really great that they're already spending time with him. It sounds like you're a teacher, so starting with half-days won't be possible. But maybe you could "practice" the week before school - get up at school time, go through your morning prep, and bring him to one of your caregivers (or have them come to you, if that's how you're doing it). Then run an errand or something not too long. Each day lengthen the "away" time, so the next week won't be such a shock.
Maybe make a little picture album for him to take with him. And of course a lovey - even if he isn't particularly attached to anything yet, find something small he can sleep with and make sure his caregivers give it to him at naptime. Waking up without mommy around can be one of the most confusing things for babies.
He's pretty young, but peek-a-boo and hiding favorite toys under a blanket are good games to help him understand object permanence.
Have some faith in your child too. He WILL adjust, and his relationships with his family will be that much stronger for it. It will be rough for a while, but consistency will help both of you through it. And I do mean consistency - consistent mealtimes, consistent mommy play time, consistent bedtime.
I really wish you luck. I was fortunate enough to be able to stay home with my daughter, but if I HAD needed to go back to work, it would have been awful. She was super needy (still is, for that matter!) and I didn't have any kind of support network then. I hope you get lots of good ideas!