What you also have to think about: just read here in the thread, there is quite a few people that say they don't want to race at all on a given day if the races suck in their opinion. So by adding a 4th race for sure you are also growing the daily player base a bit. Probably not by the 33% needed to keep it even in the short term, but possibly in the long term.
For example personally I really do want to improve with the Gr. 4 cars at this time, so I really want a Gr. 4 race every day. It used to be that way until a few weeks ago. But now with Gr. 1 always removing the Gr. 3 or Gr. 4 race you don't have Gr. 4 races half of the time, and even if there is one it is sometimes on ovals, where I also just don't want to race.
tbh this is getting a bit frustrating for me and in the long term I might even quit because of that, if the game doesn't really give me an enjoyable way of improving with the Gr. 4 cars (yes I could drive offline or something, but this is not really fun for me anymore).
In my opinion you could discuss whether growing the number of daily races beyond 4 races makes sense (e.g. I'd also like to have a regular one make with a faster N road car, these are always quite popular with the people on my friends list and myself), but having a fixed Gr. 4 and Gr. 3 daily every day is a must have, as well as the One Make N car is a must have. So that leaves dropping Gr. 1 or introducing a 4th race.