You may be surprised what someone can get done in 3 days a week. You’ll have a as well a grateful employee, who it sounds like has a range of interests (this May mean even more critical thinking and solution creativity for you).
Also, it’s at least been my experience, that when we grow out of really small teams, it can take a while to define really well what a role should look like. This is further then an opportunity to test this 4th employee role without paying for downtime during that definition phase.
And who knows, maybe he falls in love with your project even more after he starts and you can re-approach full time work in the heat future.