To calculate the total magnification:
Remember a microscope has two lenses.
1. The lens that a person looks into is called the ocular lens
2. and the lens nearest the specimen (pictured) is called the objective lens.
So in your case the objective is 4x and the ocular lense is 10x
Total magnification = power of the objective (4X, 10X, 40x) and multiply by the power of the eyepiece, usually 10X.
Total magnification => 4 X 10 = 40x
=> 10 X 10 = 100x
=> 40 x 10 = 400x