Can I install 2/4 shelves on three of four vertical standards without using a horizontal hanger? Verticals will be going into studs.
(I am familiar with the image posted in the question: some 'shelf systems' include a horizontal bar across the top, and the uprights have a notch which is able to hook onto that crossbar.)
The real reason to use that crossbar at the top is so you can carry the load between studs. Since you are able to screw your uprights directly into studs, that crossbar at the top is not really needed.