That alien doesn’t even have a numeral for 4, so it wouldn’t have a word for it either. If it speaks English, uses base 4, and uses the same words we do for numbers, it would sound like this when counting rocks: “One, two, three, ten, eleven, twelve, thirteen, twenty…”. But when it says “twelve”, it means the same thing as when we say “six”.

The joke: In any base X, the Xth number is always the first number to require a second digit. Base 4 goes 1,2,3,10,11,12,13,20,21,etc. Base 7 goes 1,2,3,4,5,6,10,etc.