Definetly... call a lawyer and HR. He may be able to take you off of the insurance, not because of any "divorce" but simply because he can. My insurance doesn't automatically cover a spouse or other dependent. I have to "opt-in" for them.
If you've been separated for four years, why have you not filed for divorce? Get the ball rolling! If you're entitled to it, you can receive spousal support and then buy your own insurance. Remaining married puts you at risk financially... if he cheats on his taxes, you could be liable for them (You can file an "innocent spouse" declaration, but who wants to do that.) He can also damage your credit rating if you have any joint accounts (If you do, get your name off of them, ASAP!)
Don't try to talk to him, or her, except through a lawyer.
(Depending on the state, your ex's girlfriend may be trying to claim a common-law marriage.)