These are hexadecimal numbers-- they are base 16. A single character represents numbers from 0 to 15. 0 through 9 are represented by the usual digits 0 through 9. Decimal 10 is designated A, decimal 11 is designated B, etc., and decimal 15 is designated F. So 0 through 15 in decimal correspond to 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F in hexadecimal.
You can store a hexadecimal number in 4 bits.