Unless there has been some major change to the ATX standard that I am unaware of, you should be able to use a modern PSU with high 12v and lower 5v on P4 motherboards in theory.
Back when the 20p power connector became a 24p power connector, the 4 extra pins would overhang when the power connector was properly seated. For a while, PSUs had 20+4 instead of a solid 24p connector.

However, chances are that the PCB area on the motherboard where the extra 4 pins would overhang, is occupied with components, making a 24p power connector physically not able to fit (best to not try to force or wiggle it in anyway if something is indeed in the way).

There are 24p to 20p adapters out there, but build quality may vary wildly. You may also need to find SATA to molex adapters as modern PSUs typically don't come with those anymore. My experience with such adapters is that it's usually the molex bit that may be crummy (always be alert when plugging one of those adapter molex connectors in). The sata bit usually fits relatively secured because of how SATA is designed.

Tetrium wrote on 2023-02-10, 20:28:

There are 24p to 20p adapters out there, but build quality may vary wildly. You may also need to find SATA to molex adapters as modern PSUs typically don't come with those anymore. My experience with such adapters is that it's usually the molex bit that may be crummy (always be alert when plugging one of those adapter molex connectors in). The sata bit usually fits relatively secured because of how SATA is designed.

You don't need any specific 20p adapter. There's a bunch of 24p risers which come as 20+4 at the end connection. And quality won't matter much on P4, unless you specifically looking for something super-sketchy.

Pentium 4 was the first platform that leveraged the 12V-heavy ATX2.2 architecture that still is used today. There's no need to go for excessive >500W PSUs. A 300W PSU should be more than enough. Instead of going for unncessarily high Wattage, rather invest in a better quality low-Wattage PSU.

dionb wrote on 2023-02-10, 23:28:

Pentium 4 was the first platform that leveraged the 12V-heavy ATX2.2 architecture that still is used today. There's no need to go for excessive >500W PSUs. A 300W PSU should be more than enough. Instead of going for unncessarily high Wattage, rather invest in a better quality low-Wattage PSU.

+1

Powering my P4 with a picoPSU 120W.. no 3D accelerator though.. only on board VGA. Works fine

The Serpent Rider wrote on 2023-02-10, 22:30:

Tetrium wrote on 2023-02-10, 20:28:

There are 24p to 20p adapters out there, but build quality may vary wildly. You may also need to find SATA to molex adapters as modern PSUs typically don't come with those anymore. My experience with such adapters is that it's usually the molex bit that may be crummy (always be alert when plugging one of those adapter molex connectors in). The sata bit usually fits relatively secured because of how SATA is designed.

You don't need any specific 20p adapter. There's a bunch of 24p risers which come as 20+4 at the end connection. And quality won't matter much on P4, unless you specifically looking for something super-sketchy.

Maybe the ones I got were super-sketchy 🤣
I'm talking about the molex adapters here btw, just to clarify.

Molex won't be any sketchier than typical connection on authentic 300-400w PSU crap of P4 era.

The Serpent Rider wrote on 2023-02-11, 11:34:

Molex won't be any sketchier than typical connection on authentic 300-400w PSU crap of P4 era.

The female plugs (I mean the ones that does not have the pins sticking out, so it's probably a male plug) on PSUs were usually much more sturdy. I just recalled the ones going from sata 2 molex should also have the female molex plug which is more sturdy, so indeed it shouldn't be much of an issue. It's mostly fan adapters that were a bit questionable, my bad 😅

The build quality of PSUs in systems build in the P4 era is another thing, it could vary wildly.
Back in those days PSUs were really a crapshoot with many people skimping on their PSUs, so what ended up in a build that I found could contain a PSU of wildly varying build quality. This is something which has definitely become much better. I've had much less luck with other modern components though with multiple brand new modern parts dying for no apparent reason.

dionb wrote on 2023-02-10, 23:28:

Pentium 4 was the first platform that leveraged the 12V-heavy ATX2.2 architecture that still is used today.

This. It shouldn't be hard to find a decent PSU for a Pentium 4 computer nowadays.

The 24-pin ATX connector is fully compatible with the 20-pin one. If it isn't 20+4 and there is obstruction in the motherboard, this can be solved without needing any adapter by cutting carefully the extra 4-pins from the connector.