I am building a desktop computer for only the second time, and just discovered that the case's 4-pin power connector, coming up from the bottom, is simply not long enough to reach the socket near top left of the motherboard. Not even close. It's a full-sized roomy case and the board is considerably smaller.

Before I go spending a little money on an extension, I wonder if I have to plug in that cable anyway for the computer to run. I don't have the experience to know. I'm no gamer or any other sort of power user. I don't do much other than use office suite programs and the internet.

That 4-pin connector powers PCIe so it may or may not be absolutely necessary depending oin what shares that bus. The most obvious power hungry device on that bus is Graphics which may not be an issue for you if you don't need 3D Acceleration and/or depend in IGP in your CPU instead of a discrete card. There is a possibility you may need it for proper sound reproduction if your Audio device has power to drive headphones and you use those. Built-in wifif may need that power for top performance.

Any non SATA SSDs will likely need it. Whether mounted on a sled in an actual PCIe slot or in an M.2 slot, they use the PCIe bus. They don't use anywhere near the wattage of a mechanical drive but having plenty of power available may constitute valuable headroom if you run more than one.

Bottom Line - every decent BIOS/UEFI I have seen in over a decade will report if any subsystem lacks power immediately in the POST process so you can't hurt your hardware by trying w/o the 4pin. Certainly it's just safer to have it.

I would check the motherboard manual to verify. I thought the 4 pin was processor power and 6 pin was PCI power.

Yes that 4 pin was introduced with the P4 as I recall. It is to deliver power to the cpu because of increased current loads. I doubt if the machine will run at all without it.

The system will power up it, but certainly won't boot. Fans will spin as I recall, but that's about it...

There is no hard rule sp[ec for the 4-pin. I have owned motherboards with only a 6 pin and others with only a 4 pin and others with both. There is considerable discretion left to manufacturers and they don't all agree. See here if you don't believe me

https://www.computer-hardware-explai...connector.html

Note that "other things" is not defined. The bottom line is still that no damage will occur if one is not connected. It just may deliver an error message or simply not even post. The wise move is just install it. It is very common for manufacturers to cheap out on power supplys when in fact they are the single most important component in a PC. Literally everything depends on power supply.

Smart move? Spend a few bucks on a serious supply..

I would just take my soldering iron, some wire and heatshrink tube and extend the damn thing.

ATX specification version 2.2

OT (sort of) - I just watched a video that featured a SeaaSonic case that will work with any power supply but has a special feature that works really cool with their newly designed power delivery system that I hope catches on. Their base PSU strictly delivers 12v which requires a distribution center that mounts just under and right next to the motherboard. This eliminates long runs insuring compatibility with any and all motherboards and nearly eliminates complex and messy cable management, improving appearance and airflow.

That SeaSonic case is here - https://seasonic.com/syncro-q7-series

The video I saw that SeaSonic case featured in was this one - https://www.youtube.com/watch?v=-tPwmK5CZUg

but alas it is far less interesting build than the one duplicating Linus Torvald's newest PC, which is here for anyone interested -

https://www.youtube.com/watch?v=Kua9cY8q_EI

The SeaSonic build though can be fast forwarded to see the end product which is meticulously and effortlessly clean.

That's great, but one assumes such a case is expensive.

This.

If there is a connection on the board you will need to power it. So you will need an extension cable.

True but one doesn't have to buy the case. The power supply AND distribution module comes as 3 kits, starting at 650 watts @$119. TheThe 850 watt model with module sells for $189. Probably many here, especially those not from the Electronics field, consider that expensive even though that's competitive with other high quality PSUs, but it's really not especially over expected lifespan, not to mention system reliability from rock solid votages.

PSUs are the single most important piece of gear in any electronics gear but they aren't "sexy" for bullet points and offer a convenient cyt costs point for OEMs,. so few outside Electronics realize this. A 650 watt PSU of 90% efficiency will consume less electricity than an 80% efficient 500 watt PSU. That really adds up on your bill. So which is truly more expensive?

A serious contender for "most important" is thermal considerations and higher quality PSUs have better cooling AND this one promotes both distribution of heat over greater area because of the module AND substantially improves airflow due to cable cleanness which reduces the heat exposure of your entire PC.

If that doesn't seem of value to you then "expensive" might apply in your use case, but if you keep a case and PSU for 15+ years, and care about maximum performance and reliability, it's actually rather trivial. Each must decide for himself, of course. It's just best in my view to know what you're choosing. I'm an EE in the field for over 50 years so I have some info to share... that's all.

Just be sure you are looking at the right 4-pin connector and not one that is a detachable part of a 20+4 pin ATX power connector. I got a bit confused by that once when attaching a new power supply to an old ATX motherboard with a 20-pin power socket. It was fortunate that the "+4" connector on the cable wouldn't reach the CPU power socket.

What are the wire colors on that 4-pin connector?

It's pretty hard to know what every PSU manufacturer uses for color-coding but there is a way to figure it out regardless. 4-pin connectors for modern extra board power (extra to modern 24pin system power blocks) have 2 feeds, 2 x grounds and 2 x 12v rails. This is the same on extra power for graphics cards - pairs of grounds and 12v supply. I have made the mistake of inserting improper polarity with a connector that had poor key-shaping. Key shapes are usually rounded corner versus square cornered. Now I check visually for corner keyways and if there's any doubt identify grounds with a simple continuity checker.

Incidentally proper corner keys and color coding are another argument for buying substantial PSUs.

True, but if the answer came back, "Yellow, orange, red, and black," then I could be pretty sure that this was part of a 20+4 pin ATX power connector.