An array substrate, its fabricating method, a display panel and a display device are disclosed. The method includes forming an active layer on a substrate, forming a gate layer on a side of the active layer facing or away from the substrate; forming an interlayer dielectric layer on a side of the active layer away from the substrate, which includes a first, second, third and fourth film stacked in this order in a direction away from the substrate; forming a via hole extending from the interlayer dielectric layer to the active layer; forming a source and drain layer on a side of the interlayer dielectric layer away from the substrate, and in a region not covered by the source and drain layer, removing the fourth film in the interlayer dielectric layer at a same time as forming the source and drain layer.