3 bet is calculated as the percentage of times you 3 bet when given the chance to do so.... As you can see from my stats for my current session I do this 9.4% of all hand where there was a PFR before me,
That equates to 1.1% of the time from MP, 5.7% from CO, 9.8% from button, 10.2% from SB and 11.9% from BB.
The 4bet percentage is all of the times you raised a 3 bet when you had the opportunity to do so... As you can also see above I 4bet 13% of the time that I am 3 bet, this varies by position also.
(Just for clarity in case anyone was unsure then the BB is considered a "one bet", an opening pre flop raise is considered a "two bet" and a re raise here is the "3 bet" and so on...)
You can also get 5b % but 100BB deep this is rare as it is usually a shove, your range is already narrow and you are not going to see this often... (Eg. In the 3.5K hands above I haven't 5b once without shoving)