Four year gap here and I'd definitely recommend it. DS1 understood everything from the beginning of pregnancy to now, and was really excited. Adored his little brother from the moment he found out he was coming and there has never been a shred of jealousy. He forgives his little brother being silly, forgives him when he's violent, is very calm with him and always has time for him. They play together still, just probably in a different way to if there was a two year gap. It's still absolutely lovely though. For me it has been easier, cos eldest doesn't need as much attention and is at school, giving me time for toddler groups etc.