Hazel says that Fiver is called Fiver because he was the 5th of a litter. But in the authors' notes he says that rabbits can only count to 4 and anything over 4 the consider a lot
This is not an error. Richard Adams goes on to say that there may have been many more than five rabbits in that litter (I think he mentions the possibility of up to 13 rabbits), and that the rabbit word which he translates as "Fiver" literally translates as "one of many". There is thus no inconsistency between this use of "Fiver" and the idea that rabbits can only count up to four, unless one insists on interpreting "Fiver" too literally.
Yes, but it says as a footnote on page 19 that any number above four is called "hrair" or "a thousand". Fiver's actual name was Hrairoo or "Little Thousand".
OK, I got it slightly wrong in saying "literally". The literal translation, as you say, is "little thousand"; but I don't think I'm wrong in saying that this broadly translates as "(one of) many".
Fiver's name is actually Hrairoo ("little thousand") because he was after four, and anything bigger is "a lot/a thousand". One of the rabbits knew that 5 was not neccessarially after 4, but it was just plain bigger than 4, hence Fiver, even though there were probably more than 5 rabbits in the litter when he was born.
OK, this is just a no brainer. Fiver's rabbit name is Hrair-roo, right (it's been a while since last I read Watership Down, so I'm hoping that's right)? The translation is "little thousand". We only know he's fifth because he came after the first four. Hazel would have been saying "hrair" rather than "five". To him, it could have been five, five hundred, a thousand, whatever. We get "five" as a translation to keep from confusing us because if Hazel were to say "thousand" so early in the book it would have left us with little question marks floating over our heads going "even rabbits can't have a thousand babies!" not to mention that logically Fiver had to be the fifth, like I already said, because he came right after the first four.
I may not explain this well but I will try my best...the English language fails me sometimes;) "Fiver" is an approximate translation. Literally his name means "Little Thousand". This would not make for a very catchy name to go calling him "Little Thousand" all the time. The author probably wanted the names to be in English. Take Bigwig for example. His name translates literally into Furhead, and the approximate translation is Bigwig. "Fiver" just came about because rabbits can only count to four, so anything else might be the "next number"in line (if that makes sense), and in English/other Human language terms, five is the next number in the series, hence Fiver.
Also, when their rounding up rabbits, a rabbit that Hazel vaguely remembers comes. Hazel then remembers the five days(Or weeks, I forget) he spent with him in the rabbit hole. Uh-oh rabbits can't count past four...so how did Hazel know it was five days he spent with him?