Well i took an online course and the guy giving it said it was kind of a quick way of calculating the percentage of hitting a project, and that it works with 10 outs or less.. with more than 10 it becomes les accurate.
I found this post where some people answered and kind of go over the math.
https://www.google.co.ve/amp/s/amp....oob_question_math_behind_the_rule_of_4_and_2/
Not really sure if its completely accurate, but i think it explains it well.
And also, i believe you only multiply by four if you are going all the way to the river.. i mean all in on the flop, yeah, thats x4.. if you're only paying to see the turn its x2. Meaning, its x2 per street.
Hopefully someone will correct me if i got it wrong