don't go for 4096x4096
1-most cards don't go over 2048x2048
2-to map a sphere you need a map with the lenght double of the height (so it would be 4096x 2048 at least)
IMO it is better to learn the very basics of uvmapping in Lithunwrap and converting to FS in order to create a skybox, since there are different techniques to map a sphere, depending by what you want to obtain. For example, it is possible to use more than 1 map (suggested).
It isn't difficult to learn but you need to know the basics of uvmapping in Lithunwrap, how to create a sphere and flip the faces (you can do that both in Lith and TS) and how to set a hierachy in Truespace in order to convert a model.