You can use the Pythagoras theorem for this since a right triangle is formed.
[tex]c^2 = a^2 + b^2[/tex]
C = 12
A = 4
B = x
Input values:
[tex]12^2 = 4^2 + x^2[/tex]
[tex]144 = 16 + x^2[/tex]
[tex]128 = x^2[/tex]
[tex]x = \sqrt{128}[/tex]
[tex]x = 11.3137...[/tex]
So, about 11ft.