I think the way to solve this is with the TextureGroup LODBias setting. You make a platform-specific override DefaultEngine.ini and add stuff like this to it:
[SystemSettings]
TEXTUREGROUP_Character=(MinLODSize=256,MaxLODSize=4096,LODBias=0)
Setting the LODBias to 1 or 2 for mobile. A bias of 1 drops down from 4096->2048. Now I can’t remember exactly where the file has to be or what it has to be named for this to work.