Vinayak, you should work out some "salary-type" compensation whether that is a stock grant (as Robert suggested) or deferred salary (to extend your runway), and then put in place pretty much what you proposed (4 year vesting w/ 1 year cliff). There's no real way to know what will happen in the relationship. If it's someone you know and respect then you're further ahead than many. What the vesting makes possible is trying this out to see how it works. If it doesn't work you can simply terminate the relationship and pay whatever deferred compensation has accrued at some point.
This structure allows both of you to "try this on for size" and see if it fits and it allows the co-founder to buy into the idea and make it his/hers (important for the co-founder), and to start adding value (important for you to see return).
As for the equity position, pick a number and discuss it with the co-founder. Everyone is different as to their desire for equity vs. salary, some want all equity and are willing to invest, while others are more biased to a salary with a smaller equity stake (the latter is a possible issue in ensuring there is proper motivation for success).