I'm just about to dip my toe into the world of the newer ATMEGA4809 chip and have got my hands on a couple of DIL packaged devices.
From my research, I know that I need to buy/build a UPDI programmer in order to use this part. There are a few designs out there and the one I'm looking at is a simple one using a Nano and a 4K7 resistor.
However, I've also come across more complicated programmers that generate a higher programming voltage - HV and PCHV modes?
When would I need to use the higher voltage when programming a 4809 device?
I couldn't find anything specific to the 4809 and HV / PCHV programming but that could be because I'm overlooking the obvious!
I'm guessing that the HV and PCHV modes are used when trying to reset some specific fuses?
You should never use the high voltage programming options when programming an ATmega4809. It does not support that programming mode, and attempting to use it may damage them.
Only tinyAVR 0/1/2-series parts support that (the upcoming DD-series parts also will, though they want the HV pulse on reset, not updi as I underestand); it is used on those parts when the fuses (specifically, SYSCFG0) have been set to disable the UPDI functionality - an HV pulse is used to turn it back on.
PCHV is closely related; PCHV powercycles the device prior to sending the HV pulse; this is required if the UPDI pin has been set as a GPIO pin and is being used as an OUTPUT, as it would otherwise be fighting the HV pulse.
Oh, and you don't want a 4.7k resistor with a nano running jtag2updi as programmer. Early on there was an inaccurate perception among Arduino folks that the 4.7k value was the "right" value. it's pushing the upper bounds of values that will work. I use 470 now. It just has to be somewhere between 220 and something like 5-7k to be in spec.