On the outset if you think the startup would do well, then I would suggest to join even at a 4% rate - the reason being that its better to join a more promising startup at a below market compensation rather than the other way round.

One way of coming with a good figure is as follows:

Assuming your market salary is x /mnth and the founders is y /mnt. Also, assuming that you get funding in about an year. Then finally based on the assumption that the startup equity is a function of hte risk that you take, then each founders total investment comes to:

12y + 18y

and your investment comes to:

12x

So essentially your stake in the company should be around

12x / 3*(12y + 18y) + 12x

Also, there is no basis for your stake to not get diluted and incase the startup goes for multiple rounds 4% might looks like a very high figure which the founders might end up not being very comfortable with.I would suggest, and for other reasons also, that you might consider negotiating a bigger stake and accept dilution.