Although 4 years is not always as huge of an age gap, your 1st grader should not have a 5th grader as his primary playmate. Shortly, this 5th grader will be starting puberty (if not already) and will be getting into girls. This boy watches movies and plays video games that are not appropriate for your son and they are at 2 different developmental levels. So, although this 5th grader sounds like a great kid and he's making it fairly easy for you to not seek out playmates your sons own age.... it's really not appropriate.
So, as other mom's suggested you need to do a couple things:
1. This is a GREAT time to teach him that it's not personal if someone doesn't want to be our friend, or doesn't always want to be our friend, or doesn't want to be to us what we want them to be to us. And that even though you can really really really want to play with someone, they just might not be appropriate as a playmate (these are skills that will come in handy and reduce his heartbreak when he gets interested in girls later).
2. He needs to get involved with kids his own age who share his interests at his developmentally appropriate level. If those kids are not pre-packaged on his street then you, as the parent, need to seek out experiences where he can have these social interactions. Enroll him in Tae Kwon Do, guitar lessons, gymnastics.... whatever he likes where there are kids his own age.
3. Teach him to be his best friend. There was a period in my llife where I didn't want to be alone with me. But if I'm not my own friend, I can't be someone else's. He needs to learn the skill of self-entertainment, even for extended periods of time.
I'm not sure how long your school day is... or how much homework your son has... but now is a great time to teach him the difference between "homework" and "studying". How is it that he has all this time to play with neighbors every day? between daily homework, 30 min minimum reading requirements (in most schools and if not at school, institute this at home) studying topics for school and additional topics that interest him, helping mom with dinner and making sure his room is picked up and his toys are pick up from everywhere else in the house and his laundry is put away as well as practicing any extra curicular activities and then shower and bed..... well, I'm not sure how there's time to miss this older neighbor boy?
That being said, an older wiser (gentler) neighbor can be a GREAT influence. So, your families should have monthly dinners where the boys get together. Foster that relationship so when your son is being pressured to kiss a girl or drink beer and he doesn't want to come to YOU for advice he can go to this older boy who will be sensible and give him some good "I've just been there" advice.
Good Luck!