Factoring cubic and further polynomials

Hi, just stopped lurking on mhf and made my first post.

Ok I can factor quadratics and the differences of squares. I'm basically using the factor pairs method (finding factor pairs that add up to the b term) for quadratics as it almost always works. I can expand cubic and further polynomial brackets but I cannot do it the other way around by factorizing.

I've hit a roadblock trying to factorize these polynomials. Could anyone please help me with them?

$\displaystyle x^3-3x^2-4x+12$ and

$\displaystyle x^4+27$

$\displaystyle 3x^(3/2)-9x^(1/2)+6x^(-1/2)$ and finally

$\displaystyle x^3y-4xy$