Mamachan
dun worry about the age gap. Different age gap has got its own advantage. Both young young one, grown up together. Only parents more xiong. My fren's daughter is exactly 5 years and birthday same day, her daughter adore her brother. Mine is 3.5 years not much diff from your 4 years, it is good age that they do understand what you say and know how to show their affection.
Though my kids fight daily and jealous of one another, but when they show their affection ha ha it is very heart warming. Like on Sat nite, I came home after making Aden sleep at mum's place, he woke up in the middle of the night, Charmaine took over my role and pat Aden to sleep and even sing for him.
Aden will do anything for Charmaine and if he ask for water, he will make sure I prepare Charmaine's share too.
Of course parents has to make sure we treat them fairly and also spend some quality time individually. Especially with the elder one because they only can remember ha ha for a good 3 years , they do not have to fight for attention man.