One little piece of advise I can toss out is to realize people like different sex play or kink differently. So you may discover some kind of sexual activity with one of your partners have no interest.
But you are feeling left out, so I agree with the others that you should talk about this with your partners. Maybe you all can work out a way for you to fit into their play in a manner that you like.
But it sounds like there is a lot of potential for you four to have a great relationship. I wish you the best.