If you have Studio One v4 prime, it will not work. You cannot add VST plugins to Studio One v4 prime.
If you have Studio One v4 Artist, you need to purchase the VST adapter before you can add VST plugins to Studio One v4 Artist.
if you have Studio One v4 Pro, then you should be able to install VST plugins.
On a side note, if you do have Studio One v4 Artist, it’s actually less expensive to upgrade to Studio One v5 Artist (which does allow adding VST plugins) than purchasing the VST adapter for Studio One v4 Artist.