Some great input on this thread. I'm going to throw some petrol on the decision making fire however, as this was an issue which affected my mainboard choice. And it is, my existing ATX power supply didn't have a 4-pin ATX power connector. Now, certainly adapters are available to try and squeeze this from a 12V molex drive power connector, but I was weary with this approach. There are mainboards available which simply do not need the 4-pin ATX power, and I went with one of those (ASRock X570 Pro4).
The spec of the Asus Prime x570-p shows:
1 x 8-pin ATX 12V Power connector(s)
1 x 4-pin ATX 12V Power connector(s)
Which means, as can be seen on the photos on the Asus site, an extra 4-pin ATX connection is required (just left of the 8-pin connector on the board). This is not to be confused with the PCIe 12V power cables, which are designed for graphic cards. The polarity is reversed between these, and they are anyhow keyed to prevent such misfortune. However, start messing around with 1-euro adapters from Amazon, and if you end up connecting the 12V output of the PSU directly to ground, it will have been your own fault and no dodgy cable manufacturer is going to be held liable.
Plus, there were enough photos of such adapter cables literally melting and burning out due to inadequate gauge of wire (even if they had the correct polarity), to send me looking for a mainboard which didn't need that at all. And there are several, just check for 4-pin ATX power connector in the specifications, if it has the connector, it needs that output from the PSU.
Does your PSU have a 4-pin ATX output? You'll have to check. If so, it will be a white/black/clear plastic connector with exactly 4-pins. If it's red or marked "PCIe" or "graphics" or "GPU", then no, it is a PCIe power (4 or 6 pins) and not the one you need.
Some pics here: http://www.smpspowersupply.com/connectors-pinouts.html