dimension 4550 motherboard front panel connections

Dell has never published the specs on their proprietary connection.  I've seen a post where someone traced it out but don't recall where it is.  Do a search on this section and you may get lucky and find it.  Otherwise you are basically on your own to figure out which pins are the front panel power connection.

http://www.lnmdesign.com/400sc/front_pnl.html#FNT_PNL    this is for another model but may apply to yours, it is better to verify by tracing your wires from the front panel.

Between 2400, 4550, and current chassis, there are at least 3 distinct pinouts with nothing in common.  Even the connector style changes.

 

I hope you still have the old front panel.  It's quite an adventure if you haven't done it before, but trace the wires from the power button to the MB plug.  Hint:  one wire is ground, and touching any ground to the other wire activates the power button function.

 

At worst, buy a new front panel board and cable from Dell parts.