The old ballast is a instant start ballast while the new one is a rapid start. They do make the bulb ballasts or you could use a one bulb and a two bulb ballast. I'd be a bit concern with using a four bulb ballast with out specific documentation. I've seen the end result of a four bulb ballast in a three bulb fixture and the light out put between the three bulbs was noticeably different. I doubt those ballasts were listed for three lights though
A rapid start ballast can be wired in a instant start fixture but it has to be carefully rewired per the ballast diagram. Also, a rapid start ballast has a lag in lighting up. that could lead to a customer complaint. I'd suggest to advise the customer of the start up before hand. That's additional labor meaning added cost to someone. If possible, I'd return it and get a correct replacement ballast if practical
