4x, 10x and 40x are all magnifications on a microscope. The number denotes how many times larger the specimen appears than it would if you were looking at it with your unaided eye. So, for example, a specimen that is 10 micrometers wide would appear to be 100 micrometers wide when viewed at 10x magnification.
The main difference between 4x and 10x is the amount of detail that you can see. At 4x magnification, you will be able to see the overall shape of the specimen and some of its features. However, at 10x magnification, you will start to see much more detail including smaller features that might not be visible at lower magnifications.
40x is generally considered to be the highest useful magnification for light microscopes as going any higher tends to result in a loss of image quality due to diffraction. However, there are some techniques that can be used to overcome this such as using dark field microscopy or phase contrast microscopy. These techniques can allow you to view specimens at even higher magnifications while still maintaining a decent level of image quality.