First, congratulations on surviving this far. 4+ years of focus is in short supply.
This sounds like the Long Term Game Devs Anonymous (LTGDA)
Finding like-minded individuals willing to work for free on software is very hard. Many people won't find a perfect fit, and when you do, it might be just 1 person. Bottom line is that chemistry is hard to get right. Now, enforcing your 'baby' to other people makes it harder, but not impossible.
This... I've tried to gather more people to support my game projects, but from 6 attempts, only one, an old school friend, is the only one who stick to the development for more then 6 month (already a few years ). It get even harder if you try to get support for an already existing game.
Here're some tips from my experiences
1. Reduce the requirements for content creation !
2. Try to do 3d modelling yourself (helps alot to overcome the art gaps).
3. Reduce the requirements for content creation !
4. After reducing the requiments, free content or inexpensive content gets attractive.
5. Reduce the requirements for content creation !
6. Invest some money and buy stuff, after reducing your requirements, it isn't too expensive.
Really, being able to code some super cool PBR engine is okay, but it hinders you more to create interesting content than it helps...