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.