The amount they've invested is some what irrelevant, except to maybe establish a minimum valuation.
I figured that the website was the company, but thanks for confirming. You also should establish how many other people in their world could have done what you did (and be realistic here). The less uniqueness you bring to the scenario, the less valuable you are overall. This is simply a datapoint.
On the plus side, it does sound like you are a valuable player to the company.
You probably do NOT want equity though. Equity has immediate value, which means you will have to pay capital gains taxes on that. If the company is valued at $500K and you get a 10% stake, that 10% is worth $50K. Are you prepared to pay taxes on that amount? Options are probably better. (IANAL, and this is a gross summarization).
You mention there are 4 partners. Let's even say that you all are equal (and in reality from what you described I think they would value their stake above yours). If you each got 10%, that sucks up 50% of the stock and will (again, summarization) make raising any future capital more difficult.
It also sounds from your other posts that you are somewhat very green to this kind of negotiating, and that can hurt you. I would spend a LOT of time googling and reading about topics like equity vs. options and golden parachutes and the like.
Based on the few words that you've posted, I would probably look at something like:
5% stake as stock options. 1 year cliff, 4 year vest
110% market rate salary
Some sort of MBO bonus (either on top of the salary or a 90% salary and MBO plan to get you to 105-110%)
Contract assuring you 6 months severance plus 1 month for every year of employment upon termination for any reason (basic golden parachute and not terribly uncommon).
10% seems high for stock. 5% is probably more realistic. In any case you'll likely end up with no more than 75% of what any of the founders have allocated to themselves.
Given the situation you should get a salary slightly above market. This is "fair" for everyone.
The contract would be a negotiating point and you'll probably have a hard time with that one. But I would personally probably still ask for it.
A lot of this hinges on how experienced the founders are all well. If they are also new to this they may be offended and think you are over-reaching. If they've done this before, probably not. I'm also assuming that you carry yourself day-to-day in a manner that is commensurate with my suggestions. If you act like a child and have a cube full of action figures, it's harder to negotiate...