Yes! My daughter was four when her brother was born.
We had my mom come and be her Birth Buddy so she could always have an adult giving her 100% attention so she could come and go as she pleased. We played it by ear for the birth itself- it was 6:00 am when he was born, and they'd gone upstairs for some quiet by then. She came down as soon as we sent the assistant up to tell them he was born, so she got to see him still squinty and slippery and "fresh." :) Someone took some photos of her holding him soon afterward. Those photos are the most precious belongings we own <3

I was home when both my brothers were born. I was 3 for the first one, and it really bummed me out that nobody woke me up to see him come out. I felt very left out. I have a very clear memory of waking up in the morning and being told he'd been born in the night. I was pissed!

I was 6 for the second, and got to sit right with the midwife and watch him come out. It was the most amazing, wonderful thing I'd ever seen. Nothing could do more to make a girl trust that birth is safe and natural- and grow up with less fear than most of going through it herself.

The main thing is talking with your son about everything, and making sure he feels that he gets to decide how involved he wants to be at the time.
We loved reading Welcome With Love by Jenni Overend.
www.amazon.com/Welcome-Lo...pd_bbs_sr_1
I didn't realize it was out of print! Ouch $$... try inter-library loan :)