Oh what do you do with a 4 year old

William has an obsession with going up to Heaven. He was so upset with this tonight, that he had to go to bed and not get to go the Heaven. Dan and I talked to him and explained to him that if he wanted to go to Heaven he would have to not be with our family again for a very long time. We explained that it just isn't possible right now. We told him the truth and he went to bed crying. After a few minutes and him not stopping we brought him back out and asked him why he was crying. He told us that he wanted Heavenly Father and Jesus and that he misses them and wants to see them. It was very sad and I was at a loss at this point. How do you talk to a 4 year old about death and help him realize that he needs to be here until its his time to go. This has really been an obsession for him since his grandma died just over a year ago.