My understanding is that you were in love with the technology or the project, but did not pay attention to who you're working with.
Even though 4% can be too little or too much (you are the one to decide that for you), having a more clear picture earlier would have resolved it prior to you putting all those hours and commitment.
"What would you have done differently to protect yourself if someone had an idea you believed in, but they couldn’t afford to pay for any legal fees?"
It is a tough one. Unfortunately, it is hard to partner with people who you do not know well. My best advice would be to constantly report to each other how one would work to develop, implement or expand the potential business, because waiting for you to work on for months without doing much work on their own is a red flag for me. There is a ton of improvements and steps (even preparation ones) that could be done in the process.
"Is it possible to partner early without formal agreements that feel adversarial?"
It is more possible the better you know the person, but it is not advisable, as with every business and money, people change sides quickly, whenever they smell blood.