The present invention provides a mobile device and an optical imaging lens thereof. The optical imaging lens comprises an aperture stop, first, second, third and fourth lens elements positioned sequentially from an object side to an image side. The first lens element with positive refracting power has a surface facing toward the object side. The second lens element with negative refracting power has a convex surface facing toward the object. The third lens element has a positive refracting power. The fourth lens element has a surface facing toward the object side with a concave portion in the vicinity of the optical axis and a surface facing toward the image side with a convex portion in the peripheral vicinity. The optical imaging lens of the present invention is capable of shortening the total length of the optical imaging lens efficiently and has good optical characteristics.