i think you should say goodbye to him, in person, but that won't be easy. After that you both should agree to go to separate ways and never speak again. May sounds awful what i said, but it's for the best for both of you.

let him go, you don't have the right to hold him for 4 years, and just leaving him with memories. With time you may start forget him (or not), but focus on the Coast Guard.

Maybe after 4 years you both can be together again, and who knows this time for good