M.T.
At 4 1/2, he can count days on a calendar and if the baby is coming in a couple of weeks, I would tell him NOW, not a few days before - he's not a toddler. My daughter was 4 when my son was born. She always knew that the baby was coming in July, probably between dad's and mom's birthdays. If you have an exact date, mark that for him on the calendar the date you'll come home.
My daughter adjusted beautifully, no problems at all. She slept when the baby woke at night, her sleep was not disturbed. We kept her normal schedule by having her continue going to camp and daycare - I was a working mom and she would have had no interest in staying home with tired mommy and baby who did nothing, she was best off being with her friends and activities.
I would suggest making sure to continue to find time to spend alone with your son. Have hubby, grandparents or a sitter over for an hour as many days as possible, so you can have time to take your son out in the yard, read stories, bake, do a craft or play cars and legos.
Also, when the baby would start to fuss, I would say, "I'm sorry, you have to wait a minute, I'm busy with K-------" Of course, the baby had no clue what I was saying but my 4 year old heard the message loud and clear - her needs were coming first at that time and the baby was coming second;
Good luck!