Step 1: Lay down the cold, hard facts (not your interpretation) in a tactful manner meant to assist in helping him vs. attacking him. Do not issue threats or ultimatums ("I will go to the board", "I'll quit if...", etc.), that will often make things much worse--hurtful to your employees/executive team. Give your co-founder some time to determine whether those facts are true or if it's just you (~2 weeks).

Step 2: If that doesn't breakthrough, then your next step is escalating with the board and letting them do a real investigation to the merit of your claims. Ultimately, they'll likely speak to someone on his team.

Step 3: If you can't do that, consider stepping down since the opportunity cost could be very high. It's much worse for the two co-founders to be at odds and potentially blow up the company at this stage. The company is learning to scale around 4 years in (usually) and if he's a first time CEO, he's probably going to be struggling.

* Try your best to remove the emotion. Consider therapy during this time as it's going to be a bumpy road especially if you've built up resentment over the last four years with each other and care a lot about your company that you started.

Email me (click on my profile) and I am happy to help you through this while keeping it confidential.