Is it considered a sin for a fiancé to leave his fiancée, whose marriage to him has been delayed for four years, and transgressions occurred between them (touching and kissing) which the fiancé has repented from, due to his feeling of discomfort towards her because of her shortcomings, negligence, and unwillingness to heed advice?

If the marriage contract has not been concluded, then touching and kissing the fiancée is a sin because she is still a non-mahram (definition) (stranger), and repentance is obligatory. If the marriage contract has been concluded, she is legally a wife, and whatever is permissible for a husband from his wife becomes permissible from her, while observing the custom regarding the delay of consummation. Breaking off an engagement is permissible because it is merely a promise. Divorce is permissible if there is a need for it, and it is disliked otherwise. One should be cautious about being lenient in interactions between engaged couples before the contract, and it is obligatory to inquire about the girl and investigate her religious commitment and character.