How did he go from side length 4 to area of 4sqrt(3)*6 here (i know there’s 6 triangles in the hexagon, just wondering how he got the root 3)?

The area of an equilateral triangle with side length x is

\frac{\sqrt{3}x^2}{4}

You can either memorise it or derive this by considering an equilateral triangle, dividing it into two equal halves, and using the 30-60-90 properties on either triangle. Or see PrepSwift videos on this topic.