I have been through pretty much the same thing... (I'm about the same age the length of the relationship was also 4 years)

I tried to save my relationship, but I couldn't as soon as I admitted to my self that I'm in love with someone else, I felt guilty and unhappy the entire time. Also I could not stop talking to the other woman... My ex was also very emotional and can't handle stress at all, so having to break up with her made me super afraid. It was a horrible time, after I broke up with her I felt terrible (she made it super hard for me, begging me to stay with her over and over again. It was the worst night of my life). I couldn't handle that I hurt her, so we got back together after a week (advise don't do that), I told my version of B that we couldn't talk anymore (we didn't for about 1 month). Than I went on a business trip for about 4 weeks (I had go too) , B was on the same trip aswell. I left on bad terms with my ex, we where constantly fighting, but I didn't break up again because I couldn't go through the same thing again. It ended in me totally falling in love with B... and throwing all my principles over board.... After coming back I told my ex the whole story and she was surprisingly understanding... so the truth helps. Long story short B and I have been dating for 1 1/2 years now, and I'm happier than ever. My ex has a new boyfriend as well and is also happy... relationship have their time of it's over it's over.