You can't stop him from opening an LLC: but you don't have to be part of it. You did not agree to be part of this 4-person company; he cannot make you be part of it, and your husband will still have his sole proprietorship. Do not transfer any money or assets to this new LLC; and explain to him that unless he agrees to cooperate in removing you from this LLC, you will sue him based on fraud for a court order requiring him to do so and for monetary compensation (e.g. your legal expenses).