I'm still learning how to be a good dad, how can I also be a good granddad when obviously a four year old takes up a boat load of time and attention?
Don't stress about it. You clearly mean well, and that counts for a lot. Common sense and being there for your children are 99% of what you need.
You've been through fatherhood, and you're having a refresher course with your 4 year old. So you can be a good source of support for your eldest son and his partner as they become parents. You know the problems, the solutions, the things to worry about, and the things to let slide. If you live near enough then I expect baby-sitting and child-care are likely to be things you can help each other with.
As for the unusual age structure, in the short term I wouldn't worry about it. Just let your son and his family visit as usual, and tell your younger son about their new baby. Later on you can get on to the technical meaning of words like "cousin" and "uncle", but for the moment a 4 year old is going to just accept whatever he sees as normal, because for him it is. People have babies. My older brother and his partner have had a baby. Can I go out and play now?
The same goes if you have a third child. The other children in the family will consider it to be perfectly ordinary as long as you treat it as such.
It is not normal ... social abnormality
"Normal" and "abnormal" are two very slippery words. "Normal" has two meanings; one is "usual" or "common", and the other is "proper" or "desirable" (plus some others in between: see the dictionary definition if you want the full spectrum). "Abnormal" should just mean "not normal", but somehow it usually winds up meaning "improper" or "undesirable". The trouble is that people confuse the two. For example, most people drive a car to work. So driving is the normal way to get to work. Does that mean that cycling to work is abnormal? Does that make cycling a bad thing? How do you deal with the social abnormality of coming to work on a bike?
If you have another child, that may make your family unusual, but that doesn't make it wrong or bad. It is not a problem unless someone else makes it one. If that happens then your priority list is a good place to start.