I think 4 or 5 Years is a bit too much of an age gap if you want them to become friends and do stuff with each other
I think that depends on the personalities moreso than the age gap. My mum is the eldest of 5. 10 years between her and the youngest. Her and her siblings have extremely close relationships and they all live in the same town & see each other all the time.
There is a 2 & 4 yr age gap between me and both my brothers, I'm the youngest and we arnt close & have nothing in common. We are 3 very different personalities. I don't think the age has much to do with being friends at all..... what do you look for a friend? Similarities and parallels to your humour, thoughts, morals & values.