【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.