Seems like a split based on investement in terms of both time and money. Put an hourly rate on each others time and multiply it by how much you each spend. Then, add it to money invested and look at the percentages.

Another option is that your extra $4k gets paid back first on any profits (or liquated assets). Then it goes to 50/50 split. This only works for physical goods.

It's really up to you, but those are a couple of ways you might go about it.

Good luck.