SNGs

How many buying would you need if you are playing 4 tables

I'm confused about your question.

What do you mean by "buying"?

WiZZiM's FAQ is a very good one. Minimum 30, probably 50, maybe as high as 100 is the short answer, same as for single tabling.

Multi tabling shouldn't have any effect on your BRM - though if learning to multi table is having an averse effect on your winrate then you might want to play a little bit deeper / at a lower level until you're used to it.

i go with 50 buy ins

I would say 40 minimum, close to 100 would be better.