1. An image forming apparatus, comprising: a photoreceptor to be driven to rotate; an optical scanning device comprising: a light source that comprises a first light emitting element which emits a first light beam, a second light emitting element which emits a second light beam, a third light emitting element which emits a third light beam, and a fourth light emitting element which emits a fourth light beam; and a deflection unit configured to deflect the first light beam, the second light beam, the third light beam, and the fourth light beam so that the first light beam, the second light beam, the third light beam, and the fourth light beam scan the photoreceptor, wherein the first light beam and the third light beam form, on the photoreceptor, a first electrostatic latent image corresponding to a first pixel, and the second light beam and the fourth light beam form, on the photoreceptor, a second electrostatic latent image corresponding to a second pixel which is different from the first pixel; an image forming unit configured to develop, with a toner, the first electrostatic latent image and the second electrostatic latent image formed on the photoreceptor by exposing the photoreceptor with the first light beam, the second light beam, the third light beam, and the fourth light beam, and which transfers the toner image formed on the photoreceptor onto a recording medium; and a control unit configured to control the optical scanning device to form the first electrostatic latent image corresponding to a first test image and the second electrostatic latent image corresponding to a second test image to be formed on the photoreceptor such that the first test image and the second test image are respectively formed in different places on an identical recording medium, wherein the first electrostatic latent image corresponding to the first test image is formed by a plurality of scans with the third light beam and the first light beam deflected by the deflection unit, and when forming the first electrostatic latent image corresponding to the first test image, the photoreceptor is not exposed with the second light beam and the fourth light beam, wherein the second electrostatic latent image corresponding to the second test image is formed by a plurality of scans with the second light beam and the fourth light beam deflected by the deflection unit, and when forming the second electrostatic latent image, the photoreceptor is not exposed with the first light beam and the third light beam.