There can be some capability problems. CS6 has features that did not exist in the CS4 time frame. If you saved normal image files like Jpeg, PNG, Gif etc they should be OK. However layer files like Tiff, PSD and PSB can have problems. If PSD and PSB files were saved with the max compatibility option some incompatible layers may be added as raster layers so most of you work will still be usable.
OK, so does that mean that I should acquire a CS6 product alone and don't mix the CS6 and CS4.
You need to answer that question I can not answer for you.