M.S.
I just went through this too - I have an almost 4 year old and a 7 month old. Some of the answers are a little harsh in my opinion. You are just trying to do your best to protect your newborn! Its not as though you have kicked your 4 yr old out of the house :0) You have probably been home from the hospital for what a few days now? I would guess you are physically and emotionally exhausted even in the best of circumstances but with a sick preschooler and a newborn to worry about getting sick? And for the record, it is a big deal if a newborn gets a fever. Its an automatic hospital admission and in some circumstances, a spinal tap to check for bacterial menengitis! Of course you should do what you can to keep the newborn from getting sick. Lets have a little empathy people.
Anyway... its great that the 4yr old got some extra gma time while he was sick... something special for him and a little distance for the baby and even a break for you. Now that gma has to go back to work, go ahead and bring him home. Do your best to keep the 4yr old from kissing/hugging on the baby and practice good handwashing and thats the best you can do.
Hang in there momma, it gets easier, I promise!!