A four-piece infrared wavelength lens system includes, in order from the object side to the image side: a first lens element with a positive refractive power, a stop, a second lens element with a refractive power, a third lens element with a positive refractive power, a fourth lens element with a negative refractive power, wherein a focal length of the first lens element and the second lens element combined is f12, a focal length of the third lens element and the fourth lens element combined is f34, and they satisfy the relation: 0.2