Honestly, if you've been together for four years and they haven't told you, don't ask. a lot of people are fine with their deadname, some people hate their deadname, everyone has a different relationship with it and it's really a personal individual thing. if they want to share, they can, but don't ask
it is pretty disrespectful tbh, but i understand your sentiment. i think you have this idea that their current name isn't their real name, which i don't think comes from malice just misunderstanding. it really isn't your place to know that, and until they're comfortable with sharing it, don't ask.
we are discussing getting married and our future. i want to be with them for the rest of our lives and have children. i feel like that’s a pretty long time and a big commitment. i’ve told them all of my secrets i just have been wondering. is it weird that i don’t know their legal name after being their soulmate for four years?