This is an interesting challenge and one that many startups have to resolve.
Generally an equity investor benefits from the appreciation of their ownership because of the increased value (valuation) of the company.
In an LLC it is possible to set up a method whereby the operating profits (losses) are distributed independent of the ownership percentage.
Since all 4 will be working “full time” in the business, you could distribute the results (operating profit) equally among them all. As the company grows and succeeds – each person’s equity would increase in value according to their percentage of this ownership.