Most P4 Dells use standard wiring, so I'd just look at the connectors and see if you have a 20-pin and 4-pin connection going to the motherboard. P3 Dells were wired differently from standard or the newer Dells.
If you find the connectors are standard, then the only "proprietary" part is whether the power socket is mounted high or low. Dell blocks the other configuration with sheet metal. You can figure out if your new power supply has the power plug in the same location simply by looking at a picture of it.
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