2-Divisible groups over $Z$

In this paper we construct nontrivial 2-divisible groups over $Z$ which are isogenous to trivial groups and prove the following:
\underline{THEOREM.} If the height h of a 2-divisible group $\{G^{(\nu)}\}$ over $Z$ is at most 4, then $\{G^{(\nu)}\}$ is isogenous to a trivial group.