in general you cannot skip updates. You will have to update from 4.2-1 to 4.2-2 to 4.2-3 to 4.3-0. Therefore it doesn’t really matter whether you do it now or later.
That being said: a critical update for a security problem in Samba was release yesterday. Everyone running a Samba-based AD domain should update right now. That update is part of the last errata updates of the 4.2-x line — therefore you really should update to the latest 4.2-x now, no matter when you plan on upgrading to 4.3-0.
 One notable exception was UCS 3.3. You could skip updating from the latest 3.2-x to 3.3 to 4.0 and go directly from the latest 3.2-x to 4.0-0. However, such shortcuts are documented in the release notes if they exist, and for 4.3-0 the release notes don’t mention one.