I don't want to presume anything, but have you calculated what your likely accumulated losses are going to be for four years, and can you sustain those losses? I don't know of a business (save for maybe Microsoft and a few other very elite and similarly monopolistic businesses) that could pay a 25% royalty on sales and even break even. If you have very rich gross margins over 50% and net margins (after all expenses) of maybe 15%, you would in that case be running losses each year of about 10% of sales. If you are in a very competitive business with gross margins closer to 25% and net margins of around 5%, you are drowning in payments, running losses each year around
20% of sales.
If you are wealthy enough, and planning for a very long run, all of this may be okay (but in this case why not just pay the existing owner off up front?) If you actually don't have the cash to pay for the business, and were planning to pay him from surplus cash the business generates, then you have a potential problem. Depending on whether business is high or low margin, a sustainable royalty rate is usually between 5 and 15% of sales, and above
15% you are challenged to survive.
Aren't you giving away every incentive you have to grow the business? Every time you double in size, you accelerate your seller's payments at the same time. That's a great deal for him and a lousy deal for you. What makes a lot more sense to me is to settle on a price, settle on an interest rate, and settle on a monthly payment. If sales in the business in a given month cannot support the payment, he should defer the payment with interest. If he won't agree to that, then at least settling on a fixed payment each month gives you an opportunity to use growth in the business to reduce your effective monthly royalty. That's a pretty positive incentive system.
As a matter of negotiating a good deal for yourself, you should be willing to walk at least twice if the seller doesn't give you a fair deal that allows you to stay in business successfully. If the seller is so greedy, or so clueless about business, that he doesn't understand why a 25% royalty on sales will drive any business into the ground, then probably you should find a different seller.