See also: Five Fives
The goal of this puzzle is to combine 4 4's with any other mathematical symbol, excepting numbers, to produce every whole number from 1 to 20.
You can extend this to 5 5's, and 6 6's, and so on.
A good strategy is to compile a long list of all the numbers you get when you combine one 4, two 4's, three 4's, and so on. This helps you chain together expressions.
Four Fours/Table of 4s - a table of various combinations of 4s
Starting with 4s:
(That is, the number 4F in hex, or base 16.)