To be blunt he may have started a relationship with someone else. Honestly men don't do that for no reason since sex is on our mind often. Men have feelings but not as deep as women do. Men who do that tend to place some sort of value in you so they won't just completely brush you off however to keep one in the friend zone just in case things don't go well with their other interest is not cool being as though you both have been in a 4 year relationship.
It isn't fair when men do this (or anyone for that matter) because you're always looked at as the runner up/second choice/if all else fails I'll settle for.......
If you love them unconditionally it's your choice whether you want to hang in there for a while and see what happens or just cut to the chase and ask them are they seeing someone else. Being in and out of a relationship isn't the end of the world. It means that things are smooth at times and rough other times . Everyone has those good and rough times.
HINT: We have to really appreciate the good times so much that when bad times comes (and they will come however it will not stay long) one can sing/dance/remind oneself in the rain about the good times. Dwelling on the good times will attract more of the same.
However if they can't be honest with you, move on but don't hold a grudge against them. Be the wiser one. love them from afar. Don't let anyone steal your time and joy. Maybe you deserve someone better. Preferably someone who can tell you how they feel about you (which lines up with his actions) and not leave one hanging and guessing.