Hi,
Generally, when you 4bet somebody, you have previoulsy "Raise 1st" ("2bet") and have been Reraise ("3bet").
Thus, when you raise the first time, your "Raise 1st" % equals the % of hand you raise among all the hands you are dealt. Ok?
Ex: If you "Raise 1st" UTG in 6max games 10% of the time, that means you are raising 10% of 100% of the hands your are dealt. Ok?
Now, when you face a 3bet and decide to 4bet, the "4bet" % is the % you raise this 3bet from your "Raise 1st" range (10%).
Ex: Same "Raise 1st" as before, you are 3betted. And you will 4bet, say 30% (random choice by me) of your "Raise 1st" range.
Here your 4bet% is 30% of your "Raise 1st" range
But what percentage those 30% represent among all the hands you are dealt:
Only 30%
of 10%, which is 3%, and this is what "4bet range" calculate!
It's more easy to associate are range to 3% than 30% of 10%, which is generally JJ+, AK+
(those numbers are just examples and not accurate, the reasoning is correct

)
Ok?
p.s.: You can find explanations on every stats here:
http://208.109.95.123/faq/afmviewfaq.aspx?faqid=155