3-4 days you will be fine. The most important thing is to breathe through your nose at all other times. It’s when you get to more extended fasts (like 6/7 days) that you may notice challenges in speaking - but this is completely based upon how much you talk during the day, so it’s quite variable.
I’m currently on day 5 (of 10, 15, 20?). My previous 7 day didn’t have any challenges in this department. I still produce enough saliva to talk normally throughout the day. For part of my job, I talk on the phone a lot.
I forgot that some people are mouth breathers. Many years ago, I focused heavily on conditioning myself to breathe correctly, and deeply, through the nose. It’s a great point that I seem to overlook since I view the world from my own personal lens, and forget that not everyone behaves in the manner that I do.