It seems the Substance TOP has an internal resolution limit of 2048x2048 textures.
If you use a Substance Select TOP it’s impossible to get a texture with higher resolution than 2048x2048, no matter how high you set the resolution of the Substance TOP.
This is a limitation of the engine currently being used in TouchDesigner: sse2_blend. There are plans in the works to move to the ogl3_blend engine for Windows OS, which allows for a maximum of 4096x4096.