My ex did a similar thing to me, although he didn't even wait 4 months. When I met him, he was in an awful state after their break-up, and until we got together he was totally hung up on her and went on about how she was so horrible. Then we got together, and about a week later they were suddenly friends again and started spending a lot of time together.
I had no idea what to do, because it seemed so odd that he would suddenly change his mind about her and want to be around her when I suspected there were still feelings on both sides. I told him that I wasn't comfortable about it, and he ignored me and started to see her behind my back instead, which totally destroyed my trust in him and ultimately killed the relationship. I still have no idea if anything happened between them or not, because he had a long history of cheating before me and then he lied to me for 3 years.
After we'd been together for about 18 months, I finally met his ex and spent quite a bit of time with her (alone and with my ex), and because of that I was more comfortable about them being together. I was never completely at ease with it, because of the way it went to start with. I think if we'd been introduced when they started to be friends again, things would have been a lot different.
I guess what I'm trying to say is that your partner alone with an ex is never an easy thing to think about if they are kept totally separate from you. If they make an effort for you to know each other though, it's less scary because they're showing the ex that you are part of their life.