If three is five, and 5 is 4, and 4is4,

What is 8

You are counting the number of letters in the spelled number.

3 - three - has 5 letters - Hence, "three is five"

5 - five - has 4 letters - Hence, "five is four"

4 - five - has 4 letters - Hence, "four is four"

8 - eight - has 5 letters - Hence, "eight is five"