Maximal unipotent subgroup of symplectic group of degree four

Definition
Let $$K$$ be a field. This group is defined as a maximal unipotent subgroup inside the symplectic group of degree four over $$K$$. Note that there is a unique conjugacy class of such subgroups, so the group is defined uniquely up to conjugacy in $$Sp(4,K)$$ and hence defined uniquely up to isomorphism.