Skin weights options for ModelImporter.
In order to import more than 4 skin weights per vertex, set ModelImporter.skinWeights to ModelImporterSkinWeights.Custom and set ModelImporter.maxBonesPerVertex to the required number. You may also limit the number of bones to less than 4, or set a threshold for the weights via ModelImporter.minBoneWeight.
Note that you must set QualitySettings.skinWeights to Unlimited in order for additional skin weights to be used.