I started my first tech startup with a friend where he had the initial idea and then I joined him to develop the same further. So I understand what you are referring to.
Since you have a team of 4 - I am wondering why are you only talking about the the other co-founder. I assume here the roles & responsibilities of the 2 techies are clear to all 4 involved.
Its always a good idea to have a clearly defined set of tasks between 2 co-founders. To define the same you need to look at what needs to get done and who is best suited to do it. Some areas like product demos/ reaching out to contacts etc can be shared. However the core responsibilities should be clear in terms of what you & the other person will "get done"
This is necessarily a collaborative & exploratory process between the 2 of you or 4 of you. If you encounter trouble on this path - its best taken head on & addressed as those issues could or rather will come back to bite you hard.
I hope this helps. In case you need any further specific intervention please do let me know. All the best.

Hi I have worked on executive compensation in multiple companies and I am presenting information based on my recent work in this area.
COO salaries in the US will range from USD 105K to USD 268K depending upon experience, location & skills being hired. This will include base pay + 401(k) benefits + Performance incentive/bonus. If you are offering equity then thats an excellent negotiation tool.
You would have to first decide what compensation positioning you want to take or have already taken wrt your existing talent at developer level. This can determine a pay range - then look at your budget & discuss with your recruitment partner if you can identify talent at that cost. Keep a range say from 75% to 90% of the compensation ratio as your target cost for a role like COO.
Take a look at tools like Robert Half - rht.com.
I am happy to help you with the details in case you want to do a more detailed analysis/study.