It’s not clear what gender the best friend is. Usually when a guy fantasizes about 3somes it’s MFF. If this is a MMF: Is your bf openly bi or at least heteroflexible? Or is this some cuckholding thing? Honestly that would be my biggest concern here, if this came out of the blue after 4 years. I would want to have known about these desires a long while before getting engaged. Cause if I didn’t then I’d feel like I don’t know my partner as well as I thought.
Of course this could as be role play fantasy for him, and he doesn’t actually want to do it IRL. You know how to get to the bottom of it all? Have an open, honest, non-mid-coitus conversation with this man you plan to spend the rest of your life with. You can even come clean about this teenage style crush, because that really shouldn’t be a big deal to talk about after 4 years together IMHO.