4 Megapixel cameras resolution is 2592 x 1520 = 3,939,840, slightly under 4 million pixels.
5 Megapixel camera resolution is 2560 x 1960 = 5,017,600*
8 Megapixel camera resolution is 3840 x 2160 = 8,294,400. 8 megapixel is commonly known as "4K", it is called 4K because the horizontal resolution is almost 4000 pixels wide.
*Notice how the horizontal resolution of 5 Megapixel is less than 4 Megapixel, this is because 5 Megapixel is a more square image than 4 Megapixel, 5MP is similar to older televisions it is a 4:3 ratio, modern TV's are 16:9 ratio. Having 4:3 ratio cameras gives you extra height in the image, this is perfect for cameras looking down from high up on a property as they see more area than a 16:9 camera.