One thing is for sure--when you're writing a letter to someone that you are angry with--you never give them ammunition. If there's anything in there that he can throw back in your face with some mean little laugh--keep that part out. I wouldn't want your letter to hurt YOU.
And that's the thing I suppose. If he wants to focus solely on himself after cheating you out of a decent relationship the last four years--what's to say that he will even listen to what you say while he's reading it? He might gloss over anything he doesn't like or agree with.
If you can write the letter, and then go back and read it later before sending it--and feel that you spoke the truth in every line--and still want to send it--by all means DO.
If there's hurtful stuff in there, too bad. Sometimes I get sick of taking care of their feelings. When did they take care of ours?