See this pic, and why 4:3 AR has more MP than 16:9? This is my Moto G4 Plus's default camera app.
The aspect ratio has nothing to do with megapixels. Part of a 4:3 image are removed to get a 16:9 resolution, that is why a 16:9 image has less pixels.
The purple rectangle is your 4:3 picture, and the green rectangle is the 16:9 picture cropped out of 4:3 by the software. 2 megapixels are just removed from an original photo.
Megapixel: A megapixel contains 1,000,000 pixels and is the unit of measure used to describe the size of the sensor in a digital camera.
Aspect Ratio: Aspect ratio refers to the shape, or format, of the image produced by a camera. The ratio is derived by dividing the width and height of the image
You can think of aspect ratio as size of picture and Megapixel count ( in relation to aspect ratio) as the area or number of pixels contained in that aspect ratio
Camera sensors in Android phones are rectangular . Android camera sensors are usually at default 4:3 ratio and in this they capture maximum Megapixels.
16:9 is not bigger than 4:3 area wise and this setting facilitates capturing photos which is more aesthetic in certain situations. 16:9 is actually a smaller area than 4:3 hence results in lesser pixels ( it is cropped by software). The sensor resolution is the same in both cases, it is just that 16:9 has lesser pixels due to cropping
That explains the difference in Megapixel count
Aside, the first two settings (with higher Megapixel count) are for the rear camera and the other two for front camera. The Megapixel count is lesser for front camera since it's resolution of lower ( invariably the case with mobile phones)
When to choose 4:3 or 16:9?
4:3 ratio is better for quality and prints, whereas 16:9 is more suited for landscape / wide screen viewing on phone or casting to a TV. A 4:3 aspect ratio means that for every 4 inches of width in an image, you will have 3 inches of height. In mathematical terms, that comes out to the screen being 33% wider than it is high. A 16:9 aspect ratio means that for every 16 inches of width in an image, you will have 9 inches of height. Unlike the 4:3 aspect ratio, a 16:9 aspect ratio is 78% wider than high - Source