Four-color printing means that four different ink colors are used to create a range of printable colors.
The "four-color" in "four-color printing" refers to the four ink colors—cyan, magenta, yellow, and black (CMYK)—used in offset printing presses and many digital presses. These four colors are combined to make a wider range of colors. (And don't worry, your book or wall art can have more than just four colors!)
RGB (red, green, blue) is another color space used for monitors and digital images. For optimal reproduction of your digital images, you should use RGB (specifically sRGB) which is the default setting for most photo-editing software.
When using our book-making tools, your RGB images are converted to CMYK at our printers. If you're using our PDF to Book workflow, you may want to consider submitting your files as CMYK. Our publishing guide has more information on color management for the PDF to Book workflow.