SFF would closely resemble MicroATX, but it's not the same. ATX is a standard form factor that keeps the screw holes, expansion slots, and rear ports all lined up to fit in standard ATX cases. HP and Dell and others are known for creating their own proprietary format that does not line up with ATX.
Look at the back of your computer. Where are the expansion slots? Are they to the right of the rear I/O ports (ATX) or to the left (BTX)? What I'm seeing for the HP 4300 Pro SFF is that it uses a BTX form-factor motherboard which (while not proprietary), isn't industry standard anymore. You'll be hard pressed to find new BTX form-factor cases.