Why are you working only 32/hr week? I have to assume that's because you have a day job with a paycheck. If you handle marketing, communications and PR, that tells me that you are not working at the executive level developing a business strategy, but rather as functional level where you execute already developed strategy.
The first question I would ask you is what do you want our of your career and financial composition?

Assuming that you are going to maintain this workload level, I think that covetable note is the best way to compensate you for your time. Figure out the salary for your role with a large corporation, then use that as a baseline for the monetary value of your time for the past four months.

In terms of equity, with a startup that already has 20 people involved, single digits numbers come to mind.