Hey, Its not easy to say the best way to unwrap your model without knowing the detail, quality, and usage you’re going for.
As last I’ve heard UE4 doesn’t support UDIMS (UDIM textures in UE4? - UE4 AnswerHub), but to be honest there’s plenty of other methods you can use just as easily.
It will depend on the quality of model your building and how many parts it comprises of. For a simple low poly stylised car you could get away with a single material using a 1k or 2k texture. For a detailed high poly model you might want to use different materials for the various parts: paintwork, bumpers, tyres, glass, chrome, seats etc. as they will all have different properties of roughness, metalness, sub-surface, opacity, etc. and you may want to add different visual effects to them in the material.
So as a start point I’d say you’d probably want to use an atlased material (where a single texture contains multiple parts of the car) and see how much of the models uv unwrap you can fit into a 4k texture while keeping the detail/pixel density you want. If you can’t fit the whole models uvs into a single 4k texture, split them out into multiple textures based on surface type (paintwork and chrome in one; tyres, bumpers, plastics in another; glass in another) etc.
You can easily change the colour of the model (eg paintwork, bumpers, chrome etc) inside UE4 using the standard UE4 material functions. You can also use material instances (a sort of wrapper you can put around the material you’ve defined) that allows you to easily change certain parameters you have in your material, for example the texture used, base colour tint, metalness/roughness/normals strength etc).
So in theory you could have maybe only one or two materials for your car from which you can create several material instances for the individual car parts which you can then adjust independently to get the look you’re going for.