Quaternary numeral system
The number four is a useful choice for a base in counting because it is the highest number that can be quickly recognized without counting each item one by one (subitizing). It is also both a square number and a highly composite number (in the same way 36 is), making it quite easy. Despite being twice as large as binary, it has the same radix economy for counting. However, it is not the best choice for identifying prime numbers (the smallest better choice being the primorial base six, or senary).