For a start Ans 1 is utterly wrong, therte is NO RED CART in a printer.
Given a choice of a 2 cart, 4 cart or a 6 cart, I'd go 6 everytime.
2 cart = 1 with black, and 1 with Cyan, (light blue) Magenta (pinkish) and Yellow (erm yello), so if 1 of those 3 runs out, you replace the 3 together.
4 cart has individual carts for each, so you only ever replace what you need to replace, BUT the printers tend to cost a little more to buy. Also total cost of a set of carts (as 1st time round they often run out almost together) can be higher, BUT you tend to have more ink overall. Despite what Ans 1 states, you CAN but individual colours NO PROBLEM, and even if you buy a pack of the 4/6 you need, you only actually open the colours you are replaceing, so to say they start drying out is totally wrong.
A 6 cart system also has Light Cyan (very light blue) and Light Magenta (lighter pinkish) for better colour rendition on lighter tones, like skins.
If you plan to print a LOT, look at the modern colour lasers, no ink to allow to dry, goot quality these days. Toners cost more, but last longer than inks.
Years as a printer service engineer.