I've been involved in 4H dogs for most of my life, first as a member and than an advisor, now I also judge 4h dog stuff. Collies (Rough and Smooth) have been very popular in the past as 4H dogs in our area. They're very trainable, generally pretty social without a ton of extra socialization required and great family dogs. I imagine quite a bit bigger than what you want but that's the only breed I have seen a ton of over the years in 4H. But now, we haven't had a member with one for quite some time. And no doubt, some of the popularity of them in the area with 4Hers was because a 4H advisor bred them. There was another club in NE Ohio that probably half the club had Belgians (Groenendaels). One of their advisors bred them and liked placing them with youth who were interested in performance and conformation. Definitely not what most people think of when they think of a great 4H dog but those kids did really well with them and a couple of those members have remained active in the breed.
IME as long as the dog doesn't have dangerous behavioral issues and does have food drive, the biggest factor of what makes a dog a good 4H dog is having a kid who really loves the dog and is really interested in working with them. My first dog, the one that got me into 4H dogs and then into all things dog was a fearful, somewhat reactive Dobe mix and I was 11. Not a dog anyone would have thought was appropriate for the situation LOL And we struggled a lot the first couple years but in the end, he turned out to be a really great dog. One of our current members has a shelter rescue BC mix that they adopted as an adult. Probably no one would have picked this dog as a great choice for a pre-teen's first agility dog - he acted like a dog who'd lived his whole life outside with little human interaction. Not fearful but wild, not real human oriented, a bit reactive to other dogs, etc. It was a struggle for her the first couple years too but now, they're just 3 standard legs away from a CATCH.
We've had a lot of breeds involved over the years and I've seen many more in judging. Labs, Goldens, Dobes, Danes, Chis, Collies, BCs, Aussies, ACDs, Beagles, Huskies, JRTs, Rat Terriers, Hairless Terriers, Irish Terriers, Corgis, the list goes on and on. Also lots of mixes. The only breed that we had that I didn't think was appropriate for 4H overall was a Fila. Toy breeds don't tend to do well but I suspect it's more the whole toy dog thing (dog isn't socialized or expected to do anything) than that they aren't good for it. And many clubs still use old school training methods, which makes dogs who can't handle correction based training unsuitable.