Hi,
you were almost there. Instead of passing a_box (which is the feature itself), just pass a_box.result():
moved_box=model.addTranslation(partdoc, [a_box.result()], vector = [0,0, 4] , keepSubResults = True)
You can also use .results() if there are several results. To select last results from compounds or compsolids, use what is in the script from this message.
Other ways of selecting sub-shapes are described in this message.
Christophe