The fourth level of exhaustion imparts this effect on a creature:
Hit point maximum halved
The Basic Rules make it clear that whenever you divide a number for any purpose, you round down:
There’s one more general rule you need to know at the outset. Whenever
you divide a number in the game, round down if you end up with a
fraction, even if the fraction is one-half or greater.
Most Challenge rating 0 animals have a single hit point, so when these creatures reach the 4th level of exhaustion, they would presumably be left with 0 hit points and die before reaching rank 6. Is this correct?
Best Answer
The creature's maximum hp would drop to zero, as you say.
When a creature's maximum hp drops to zero, its current hp will also be zero, so it will start making death saving throws (see rules here):
It's possible the DM might decide the creature does not get death saving throws. If the DM does allow death saving throws, then the creature can become stable:
The "regains 1 hit point" won't happen, but it looks like the creature could remain stable for some time -- either until its exhaustion level increased due to lack of food and water, or until its exhaustion level decreased due to rest or medical attention.