If you own part of the company you don't just give that up without there being express wording in the charter (or whatever contract/paperwork you used to create the company). The problem you have is that right now, you own 40% of nothing so you're going to pay the lawyer you talk to (rather than have them work on contingency).
I should also note that this seems like a weird time to leave a startup ... did you really think you could cash out in just two years? Perhaps you haven't been "invested" in some time?