How about arranging a dinner/get together with all four of you. Then say something like "I love you all and am very happy for you taking this next step. I hope you know that I am supportive and I also hope this will not change our friendship too much because I would be very sad to lose what we have." I would keep it fairly simple so that it feels like a NOT TOO BIG thing. If I had to guess they are probably wondering how to approach you with some reassurance.
Sounds like good people all around! And you have lots of support here should you ever have questions or concerns that arise, or just want to gain some perspective.