You will fry your 4550, or any other P4 Dell, if you plug in that adapter. Luckily, due to all the bad info floating around, that adapter won't fit on the newer 24 pin (ATX 12v v2.2) PSUs.
Here's a quick reference for Dell PSUs:
Pentium 3 and Celerons (Socket 370) and older: proprietary PSUs
Pentium 4 and Celerons (S478/LGA775): standard ATX 12v (v2.0/2.1 for 20 pin, v 2.2 for 24 pin), except for XPS Gen 5 and SFF cases (and some *S423* systems), and with the limitation of no rear power switch.
examples of Dell desktop systems with standard ATX PSU (20 or 24 pin): Dimension 2100, 2200, 2300, 2350, 2400, 3000, 4100, 4300, 4400, 4500, 4550, 4600, 4700, 5100, 8200, 8250, 8300, 8400, 9100. (yes, that's over 4 years of systems

) And that includes the Optiplex and PowerEdge versions of similar systems.