I went through this a few years ago.
Ran a semi-successful web / mobile app development company. We were a small team of four, hitting early to mid thirties. From the outside anyone would have thought our company was successful. Good brand recognition in our geographic region, finding clients was easy. They found us.
We paid ourselves decent salaries, paid the bills on our dev infrastructure, we all worked in the trenches, at our own pace, and we all generally loved what we were doing.
However, after about five years we got that itch. In a way we were too comfortable. Our finances looked the same year after year. We purposely weren't growing. Or maybe we just didn't want to take that risk. I guess we peaked early and got comfortable staying there.
At first no one really talked about it. You could feel it though. As we got older. Being so close to the business administration became a chore. Planning summer vacations with a small team became a chore. Riding out December became a chore. Maintaining apps over a five+ year lifecycle became a chore. Dealing with dev infrastructure maintenance became a chore. Wearing too many hats became a chore.
Life outside of work becomes more important as you get older. Family and that sort of thing.
So we decided to shut it down. Lots of emotion, soul searching, panic, joy.
Anyhow ...
I personally had the same feeling you did. Does co-founder look good on a resume? What the hell did I do for the past five years? Will I fit in with corporate culture? How do I transfer my skills to the corporate world?
What I learned is that you've got nothing to worry about.
Working on a small team, and staying profitable, and paying the bills means that you understand the value of money. You wear multiple hats. You're skilled at sticking to a budget and meeting deadlines. You had to "show up" every day. You worked the trenches and enjoyed getting your hands dirty. You worked twice as hard and twice as fast. You stayed the course for four years. You're loyal. You're reliable. You understand the entire lifecycle of building something from start to finish. And you've done it successfully multiple times over. You understand failure too, and know how to bounce back. You have an amazing combination of business savvy and technical smarts that's difficult to find. You're entrepreneurial. You've got vision. You're an ideas person. You've got real-world experience. And a track record of success. Most startups crash and burn early, you didn't, and you're leaving it on your own terms. Knowing when to exit is an extraordinary skill in itself. You understand how it all works. You haven't lost touch.
Don't scare potential employers with big fake C-level job titles. In a company of four you never really had a specific job title anyways. Use that to your advantage.
Custom tailor your resume for different roles. You've got so much to offer your challenge will be figuring out what to "leave out". You've got volumes of specific examples and accomplishments to draw from, which you can talk about for hours.
You're still young. In fact you're in your prime. You'll be a massive positive addition to any team.
Look for a larger (than yours) company that hasn't lost their entrepreneurial spirit. You'll be right at home. Don't settle and don't be surprised if five or six years from now you get the itch to embark on another startup.
It's all good.