I've read most of the posts on this thread and honestly I think it may actually be the most ridiculous thread I have ever read. I am actually mad for you OP, you do not deserve the criticism you have got.
It is absolutely perfectly sensible to plan for your SS to not stay over that weekend. If you are at home that weekend then you can see him on the Saturday and he can meet his new sibling. Dad can look after him and he can be made to feel special and be given a lovely present from the baby and go back to sleep at his mums. We have to look at reality, new babies are often up all night in the first few days, if OP is in pain, struggling to feed and hormonal (all of which are quite likely so soon on, especially first time around) she will need her husband to be up with her. If she had had a long birth then they will both be knackered, those first few days are hard for everyone. Is this really the right environment for a 4 year old who doesn't sleep well to be in? Wouldn't it be better if he came, had attention, felt special and then went home where his mum will be fresh and be able to give him more attention? Visits will continue as normal the following weekends.
One night is not going to break a 4 year old, an upset stepmother who is struggling after birth and a father who is grumpy and may not have slept in days is much more likely to upset him. 4 year olds are full of energy and not always the easiest themselves (I know, I have one).
OP, it may be worth talking to your husband about how he will cope with looking after all of you if you happen to be out of action, for example if you end up with an EMCS. I was ruined after my first birth and I absolutely do not know how we would have coped looking after a 4 year old too, especially one who didn't live with us all the time. Btw, I am a stepmother myself. The second time around for me was much easier, at least I knew what to expect, had some confidence and had a much better birth.
Maybe everyone should remember that even stepmothers are people, they have needs and giving birth to your first baby is one of the few times when you may be absolutely dependant in your DH. One night where the plans change will not damage your SS, especially if he is made to feel special in other ways.
Would people think it's cruel and evil to change and overnight visit if the OP was having major surgery? Blended families are hard for everyone. Life and plans change at times and everyone has to adapt. The trick is to make the children feel secure in their positions and that they are loved. Sometimes this may mean not exposing them to highly emotional situations and to take into account the mental and physical health of all. After all, we are people, even stepmothers, not robots.