I believe Poser projects the image onto to the UV map not the obj, the  effective texture resolution applied to the obj is dependant on how the UV space is mapped to the obj (think this is called  texel density). For example if the tee shirt maps the full 0-1 UV space then increasing the texture size to 4K should increase the texture on the tee-shirt obj to 4K also. If the tee-shirt is mapped to 0.1 x 0.1 UV space the actual texture resolution on the tee-shirt will be much less (sorry bad at math). If using Firefly there is an additional setting (IIRC "min shading rate") that will also determine the effective texture resolution as this is used to "dice up" the model before rendering.