Ensure, insure, assure, secure are comparable because they all carry the underlying meaning to make a person or thing sure.
Ensure, insure, and assure all indicate a making of an outcome or event sure, certain, or inevitable as a consequence or concomitant.
Ensure in such use may come very close to guarantee.
Insure is often interchangeable with ensure, but it is also the general word for reference to making certain arrangements for indemnification for loss by contingent events.
Assure may in its more general use be indistinguishable from ensure and insure, but distinctively it more definitely expresses the notion of removal of doubt, uncertainty, or worry from a person’s mind.
Secure implies purposive action to ensure safety, protection, or certainty against adverse contingencies.