If it were me, I'd cut the strongest shoots back by a third prior to
or immediately after planting and the thinnest shoots back to the
first pair of buds. S. nigra aurea is not quite as vigorous early on
as the species proper, so I'd be less inclined to butcher young plants
immediately. Once well established, you can hack them about as
necessary, although you'll have your work cut out to try and keep a
hedge of it at just 4ft.