$\displaystyle \displaystyle \int \sqrt{4 x - x^2} \, dx$

I really don't know how to evaluate this! I cant get it into the form of any of the inverse trig functions. Im trying to learn the trig substitution method. Could this work on this? Would the trig sub method ever work if the integral was in the form of

$\displaystyle \sqrt{x^2-a^2}$

What method do I use to solve this?

Thank you very much

Heres what I've tried so far

$\displaystyle \int \sqrt{-(x^2-(\sqrt{4x})^2)}dx$

let $\displaystyle x=\sqrt{4x}\tan\theta, dx=\frac{4}{\sqrt{4x}}\sec^2\thetad\theta$

now, I have

$\displaystyle \int \sqrt{-(4x\tan\theta)^2}\frac{4}{\sqrt{4x}}\sec^2\thetad\ theta$

How am I supposed to remove whats in that root? Regardless of what trig function I use, there is always going to be a negative number in it!

Where do I go from here?

Thanks!