I'm a tech co-founder, 4 years in. I don't feel much passion for my work anymore, what should my next career move be?

That's a sad experience but not rare unfortunately.
What you are going through is very common, two people get their similar skillsets together and develop a product with no clue for preparations into what the future will look like for the company and for them individually and as a team. I would not necessarily quit but revisit the contracts and agreements that you must have had in place and maybe try creating new positions for the both of you where is the a middle ground, add a third person to be liason and conflict resolution and in charge of what you two clash the most. Focus on what strategies your company can rely on, and what each of you are good at and happy to do. Passion can come and go depending on your mood and situation so don't make judgement or permanent calls based on what you think you are passionate about that moment. You might think is not a moment thing, but you make decisions based on how you feel at any give moment. Whatever you do, working with another company you will be working for another person.
Is it lack of respect that you have? Maybe you don't enjoy working with the person so consider who is better at the current roles and determine what leadership changes are needed, maybe both need to step down. Take your board into consideration and offer them a job or hire from outside. Become an evangelist for your product.

If you haven't already, you might want to look into external help for your strained relationship with your CEO (yes, just like marriage counselling). Relationship experts can really improve the quality of the communication and help resolve existing issues.

I don't know what your personal goals are, but you could probably try becoming CTO at another company between 2x and 5x the size of your startup.

Take some time and do business consulting work for multiple clients. It will allow for R&R and give you a view into a lot of other career choices.

I am not trying to sell you on calling me. Really, I am pretty busy with my businesses and consulting. However, I need more info before I could have a greater impact in helping you.

Ask, Ask, Ask, then Ask again.

Bonus:
Here is $10,000 worth of information for free and in a nutshell.

Concentrate on the 3 M's. There are actually 7, but 3 will do for now. These are Market, Message, and Media. They come in that order.

Who is your target market (customer, clients, buyers, users, etc.)?
Tailor your laser focused message for this target market.
What is the best media mix to get your message to that market?

Here's what you do...first, make it an offer that is so incredible that they cannot resist. Secondly, do all the work for them. Make it so easy to make the purchase now that they can do it virtually without effort. Thirdly, give them an incentive to act right now. Fourthly, offer an almost unbelievable guarantee. Fifth, offer a bonus for acting now. There are many other incredible steps, but these steps should help the novice to the professional sell anything.

Whether you are selling B2B or B2C, you have to focus on selling to only one person. You can actually sell to one person at a time while selling to millions at a time. They are one and the same. Don't get off track, what we call digital marketing selling is just selling in print. And that has not changed since Cluade Hopkins wrote "Scientific Advertising." Really long before he wrote the book.

The secret to success: I have had the pleasure of knowing and working with some of the biggest names in business, celebrities, actors, entrepreneurs, business people, and companies from startup to billion dollar operations. The number one reason for their success is doing what they know and love while doing it in new, creative, and innovative ways.

Ask, Ask, Ask. Have thick skin and learn from each "mistake." In a short while, the market will tell you what you need to do and who and what you need to ask. But get started now even if that just means asking a contact on LinkedIn.

While you are thinking, think big and think of something at least 1% better, newer, or different. And being cheaper is not a winning strategy.

Make decisions quickly and change decisions slowly..unless you are actually going off a cliff.

Remember these two 11 letter words...persistence and consistency. They are two of the most important tools ever invented.

Treat everybody you talk to and everybody you meet (including yourself) like each is your number one million dollar customer.

Bootstrap when possible and reasonable. Read "How To Get Rich" by Felix Dennis. Or better yet just remember the camel's nose in the tent story.

However, sometimes you just need to make a deal.

Listen, in any business you have to take some chances and some risks. Make sure you don't need a license and go for it. Remember, timid business people have skinny kids. Paraphrased from Zig Ziglar.

Best of luck,
Take massive action and never give up.
Michael

Michael Irvin, MBA, RN

What you may have valued when you started your career as a 20-something may no longer match what you are passionate about now, leading to feelings of dissatisfaction and disengagement at work. By applying concepts like job crafting and exploring passions and interests outside of work, it is possible to make your job more meaningful. And by indulging in activities and interests unrelated to your work, you may experience positive spill over effects within your career.
And truth be told, there could be any number of reasons for this sense of malaise. Not everyone wants a high-powered career. It is this third category of people, perhaps unsurprisingly, who exhibit higher performance and a greater sense of satisfaction with their jobs. It is quite possible that what drove your career in your 20s is no longer appealing.
Even if you do not find your true calling, you will at least increase the odds of finding a meaningful work experience. There has been considerable research on the idea of job crafting, where you tweak certain aspects of your job to gain a greater sense of meaning and satisfaction. Research by organizational behaviour scholars Justin Berg, Jane Dutton, and Amy Wrzesniewski has shown that people can be quite imaginative and effective at reimagining the design of their job in personally meaningful ways. One participant from Berg, Dutton, and Wrzesniewski’s research redesigned her marketing job to include more event planning, even though it was not originally part of her job. Sometimes even the smallest adjustments can lead to qualitatively meaningful changes in your work experience.
Ignite your passion outside of work. It might be a latent hobby you’ve told yourself you don’t have the time for, a personal project that isn’t related to your job or career, or a “side hustle” where you can experiment with innovative or entrepreneurial ideas on a smaller scale. Having an outlet for your passion outside of work can counterbalance the monotony of nine-to-five daily work. Think about changing your career like you would think about changing your house.

But since then, your priorities may have changed or maybe you have simply outgrown it. You can think the exact same way about your job and career. Of course, if you choose to change your career, you will want to think it through and prepare yourself before jumping in with both feet. Network with people in professions you might be interested in, get your finances in order, and test out the new career before making the change.
It can feel daunting to change everything so suddenly, but it is important to consider the option if you are truly feeling a deep sense of malaise at work. You will likely be surprised at how resilient and resourceful you are as you walk down the path of career renovation.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath