>>430160You rollover to double-digits when you hit your base number. Base 4 goes: 0, 1, 2, 3, 10, 11, 12, 13, 20, 21, 22, 23, 30....
So if you were using base 4, you'd say "10" instead of "4".
If you were using base 5, you'd say "10" instead of "5". And so on.
The numbers we typically use are called "base 10" but that's only meaningful because we already understand we're using base 10.
When in doubt, count their fingers.
