There is 3 year 11 month between my DD who is 4.6 years and Ds nearly 7 month.
Its a great gap!
DD was old enough to understand what was happening, and it was easy to involve her, she was of an age where she was happy about going off with grandparents to give me abit of a break, without any 'being abandoned or left out' issues.
She had a small period where her behaviour was off, but she was back to normal pretty soon (to be expected, it is a big ajustment for them!)
Now it is lovely, she adores her baby brother, always saying "oh he's so cute mammy" or "look mammy he's rolling, hes so clever". She is very protective of him, and enjoys making him laugh etc. She is also very happy sharing her toys with him.
He adrores her back and breaks out in a big gummy smile when he first sees her in the morning. He giggles away at her, and loved watching her play.
Another positive is that she is in school nursery, so I get important one on one time with the baby on a morning.
I know it can all change but I have a feeling they are going to be great little friends!
Also there are 5 years between myself and my brother - we have always got on very well, and still do :)
Congratulations, it will be great!