Well no, I don't know how startups work, having never been a founder or an investor, but my understanding is that startups are valued at a few multiples of ARR, 10x if there's major growth, for which in almost all cases you'll need VC money (which they don't have).

I'm going to assume that either A) there is no growth without this partnership, so the startup is maybe worth up to 1M, in which case getting up to 40% over 4 years with work and targets makes sense, or B) the original founder is expecting significant growth even without the partnership, in which case he needs an employee and not a partner (and he should pay him as such).