public Vector4 lightmapScaleOffset { get; set; }

A lightmap is a texture atlas and multiple Renderers can use different portions of the same lightmap.

The vector's x and y refer to UV scale, while z and w refer to UV offset.

Note: this property is only serialized when building the player. In all the other cases it's the responsibility of the Unity lightmapping system (or a custom script that brings external lightmapping data) to set it when the Scene loads or playmode is entered.

Additional Resources: LightmapSettings class, Renderer.lightmapIndex property, ShaderLab properties.