If it's that small, how does the guy even believe you when you tell him it's huge? That's kind of stupid right there. You say sex is boring a lot of the time, but it's been 4 years and it's not a problem. So 4 years and it's not a problem, and you wonder if it will be a problem when you're married?
You're not very good at expressing yourself, at least in this post. Everything you say is contradictory. Physical deal breakers are usually deal breakers in a few dates, or after a few times being intimate, not 4 years.
His dick isn't going to magically grow, so either you can deal with it the rest of your life, or not.
...as ancient astronaut theorists would suggest