Base-4 (aka quaternary) number system uses four symbols to code information. Most commonly, these symbols are, 0, 1, 2 and 3.
Base-20 (also known as vigesimal) number system uses 20 symbols to code information. Most commonly, these symbols are, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 'A to J' (or alternatively 'a to j').