OK, fair enough. That diminishes your bargaining position.
So the options are:
1. Take the deal as is.
2. Negotiate the deal.
3. Keep working for 3 more years (3x revenue), and make the same money - salaries you would have made at BigCo, but PLUS you still have the company and money coming in for years after.
My calculation would be around 3: how exciting is that. Also, you have plenty to negotiate around. 3x revenue can be upped. Your employment conditions can be negotiated (ie. I get to spend 2 days a week on open source software, paid). Vesting can be negotiated (4 years is a really long time).
Or calculate it like this: current annual revenue = X. If you stay alone, assuming revenue will grow a little, work for 4 years and you get (5X + the total value of the company at that point, which is likely 4X-ish) = 9X. Work for 4 years in BigCo and you get 3X + salaries. (Adjust formula to take into account multiple founders etc). If your combined salaries would be about the same as your net revenue is now, it's a rather equal deal (except for the freedom). So it depends on how your salaries compare with your income now. A lot also depends on how you estimate the longevity and growth potential of your company. If it's gonna grow and keep going for 5 years, they're cutting you a bad deal. Also consider how much you could be making at some other company, in todays job market.
It's weird that you're sure you won't get another offer.
It also kind of sounds like a talent acquisition. Do they want you, or the company? Has that been discussed?