I think that your baby will be accepted by your son, and that he will bond with his new brother or sister regardless of whether he attends the birth.
You may wish to have your child present as part of your birth experience, but there are several issues to consider.
Can you prepare your child for the medical equipment and procedures that occur in the birthing room? Would your child be frightened by seeing blood, seeing you in pain, and hearing you groan or scream?
Do you feel you would be able to fully experience your birth in the way you would like in front of your 4-year-old, or would you have to modify your behavior in order to keep the scene more manageable?
Will you be able to focus on your own needs, or will part of you be needing to attend to your child?
Do you have someone who could be a devoted caretaker for your son, ready to whisk your him out of the room in a moments notice if needed?
And finally, do you want some time to meet the new baby without worrying about the jealousy of your 4-year old?
Many hospitals have sibling classes for young child to learn about the hospital and the birth process. I recommend bringing your child to this kind of class, and seeing how interested he is in attending the birth.
Speak with your pediatrician about your child's capacity to handle the delivery. And don't forget to consider your own needs and preferences. If you decide not to have your 4-year-old present, you could have a family friend bring him into the delivery room very soon after the birth occurs or once you have had some private time with the new baby.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.