Personally I think being friends with one's ex (the myspace guy) is a good thing. Cutting it off and not being able to be congenial after some time passes is not usually NOT a good reflection on people, but really bad stuff does happen and sometimes its best for all involved to move on. In short, if you are friends with someone you are dating, then get close, and then retreat for some reason, then its quite reasonable to still be friends at some level, albeit a reduced level, and still like some things about them.
Anyway, that should not leave your present or recently ex guy feeling threatened, especially if you have been seeing each other for 4 years. By now I would think he would know you enough to have some trust.
Fooling around with another guy while dating is a human thing, not a desirable one, but human. You just need to decide what you can and can not tolerate and why. The fact that he is telling you is a good thing, what you guys do with it is up in the air and how he says it is part of that. I think the main thing is to communicate and talk about it with him in the process of coming to terms. It really is better to discuss it with him, although it may not feel that way at the time, than to discuss it with casual non-participants. The down side is that you can come onto some unpleasant and painful stuff (risk of rejections etc) the up side is that you may find out what is really going on in his head and if there is still something there for you guys (salvage 4 years or at least get closure, learn, grow, and move on). Its OK to get advice from nonparticipants, but still get some closure on it with a heart to heart with your 4 year man.
I had a BF that I had to let go after some MAJOR outbursts in a public setting with most of his friends present. We had been seeing each other exclusively for 6 months, been talking of cohabiting. He blew up at me and lambasted me in a restaurant with 5 other people present in a weird and unexpected way about some crap I had never had heard about from him. I was deeply surprised and offended and sat quietly for it to pass but afterwards talked to him about what it was that was bothering him, and set a boundary for him to never do that crap again in public without having had brought it up with me as it was no place to air that kind of emotional stuff.
He did it again 6 weeks later and after 20 mins of discussing it privately I told him I didn't want to see him anymore. It looked like an abusive situation, he was not coming clean with what was bothering him, and I found myself in the weird position of having to make hard choices to keep any respect for myself.... Very weird stuff... and NOT pleasant. Then a year later I found out that it was just some drama thread he was spinning publicly because he was fooling around with one of the people present both those nights of the outbursts and telling that guy that the two of us were not really serious and there were a lot of issues between us... playing the other dude because the other guy no longer wanted to "see him" since the BF and I were now seeing each other regularly now. The other dude actually came up to me in a grocery store line and appologized to me about a year later, that was when I finally understood what the hell was going on with it. In any case... I talked to the BF, tried to resolve it, set boundaries for his unpleasant mode of interaction with me, then he violated those... we talked again, and I punted it, not knowing exactly why It was happening but that it was just not right in a major way.
The long and short of this is that the main thing to do with the person you have an investment in time and an attraction and relationship with is to communicate. It only goes down hill in the long term if you move in with someone that makes you ga ga and then find that they are NOT honest, open, and communicative, or that they are manipulative and distasteful. Those make for some very painful separations. If they are not all of those things then don't get involved as a couple and just stick with being friends or sex buddies if you are the type that is comfortable with that, but set your expectation appropriately in any case.
Be safe in any case... what might seem like an exclusive or monogamous involvement often is not.