Can designs created in BERNINA embroidery software V6 be saved back to earlier ART file versions (e.g. V4 or V5)?

In principle yes. However, there are limitations with objects created with the new functions of the BERNINA embroidery software V6, e.g. morphing, star fill, sculpture effect, meaning that such objects will be rescaled without stitch recalculation, and reshaping is difficult too. If your design contains such elements, you'll be notified with a message saying that objects were found that would not be supported in the output version.