Seed grown is good because it explains your tree. Citrus are polyembryonic or monoembryonic. I suspect the seed your tree grew from was polyembryonic and four of the embryos grew into trees. No, you can't separate them. You can either cut 3 stems off or you can let all of them grow and have a multi-trunked lemon tree. Personally, I would keep it multi-trunked.
Its too young to bloom. I can see some thorns - when new branches grow without thorns, it will mature enough to bloom and bear fruit. That will be anywhere between 7 years and about 20 years.
PS: The plus side to polyembryonic is the tree you are growing will be a duplicate of the fruit you got the seed from.