This really has to do with what you want in a relationship. If you want monogamy, then your answer is clear. But if you want monogamy that leads to marriage, and you’ve been with the same guy for four years and there is no marriage in sight, then maybe your long distance boyfriend doesn’t want marriage and you don’t have the same goals. And if you’re just dating because you like dating, then you should absolutely date the new guy who is in town.
Whether or not you tell your long distance boyfriend that you’re playing the field, has to do with how serious you are about him. If he finds out, you probably know what his reaction will be, so consider your goals, the nature of the relationship and then whether or not you should date this new guy and tell your long distance boyfriend about him.