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”.
But the joke would only work written, not spoken first contact? Base five we would still say five just the numerals would show 10.
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”.