Four-color printing means that four different ink colors are used to create a range of printable colors in a book. The "four-color" refers to the four color plates called CMYK.
As the name suggests it involves creating one printing plate that then may be used to print any one colour. The single colour may be any one of the standard PMS colours, or CMYK process colours.
screen printing is essentially a stenciling technique used to apply ink to a surface through a mesh screen.
UV Printing refers to a commercial printing process that uses ultraviolet curing technology. Also known as Ultraviolet Printing.