We are also going through this right now. My 4 y/o is in his fourth week of pre-k. Some days, he is great, doesn't shed a tear and other days, he cries so much that we have to go get him (like today). We sent a picture in with him and I know his class has done a project based on the book The Kissing Hand.
He has a kissing hand in school that he can kiss whenever he misses us.
It's a hard adjustment for him, going from no school or set routine to full day five days a week! pre-k (all our district offers). We keep reassuring him that he isn't missing anything special when he is in school (one of his main concerns is that we are off playing and having lots of fun w/o him). I try to find out lots about their daily activities, and I do things like sing the good morning song that they sing in class when he wakes up. We walk to school with one of his classmates and he likes that too. I just talk about what a wonderful place school is and how much fun he is going to have that day doing things like going to the library or music. We also uses bribes, lol. If he has a great day w/ minimal crying, he gets a treat (stickers usually). If he has a good week, he can have a special treat (an outing, a new video, etc). Oh, one other thing we are doing is occassionally popping in to have lunch w/ him. He enjoys that.
I also feel that he is beginning to realize that if he cries enough, mommy and/or daddy will come get him. I don't want him to start using that as a tool when he is not being hurt, mistreated or otherwise going through anything than just missing us (which is a BIG deal, but still). A picture schedule is hung in the class so he knows he gets picked up after rest time, but some days, he just can't make it that long.
The main reason we wanted him to attend is to get used to the routine and to work on socialization. I knew it would only be harder if we waited another year. Homeschooling (or private school) is not really an option for our family right now. He really looked forward to going to school until he actually had to attend.
DH and I are working with his teacher daily and we hope that this will help ease the transition. She is very understanding especially since she knows he is a young four (bday was in July). If things still don't seem to be going well in a few more weeks, we will discuss options like pulling him out, etc.
Sorry this got long, but I am right there with you! Good luck with your DS and