I'm not familar with Zmodeler but I did some searching the other day. It results that Zmodeler requires you to add the paint as a tag in the end of the material name. The tag would be [prim], [sec], [ter] and [qua]. Though as I understand the fourth color is only supported by a small selection of vehicles being the cement mixer, camper and squalo. Can you perhaps try adding the [qua] tag to a material in one of the listed vehicles and see if it works in-game using setVehicleColor or the freeroam color panel.
If it works can you please upload a sample dff so that we can get the RGB code for it, as in 3ds Max we don't have access to it, only the first 3 colors and I frankly don't know where in game files these are defined.
Source: https://www.grandtheftwiki.com/Car_Colours#car4