EXOS v15.5.3.4-patch1-4 was never released. If you have it running in your network, it would have been improperly provided out of normal software distribution methods. As Stephen suggested, you'll want to use a newer EXOS release.
Note that newer releases on the
Extreme Portal do not use the combined zip file for distributing the ssh xmod (and it's now bundled in 16.2+).