What drgeoff said.
However I currently have a 4B and 1B connected via the 4B's USB C port and both are being powered via their GPIO headers.
Probably not ideal if each have their own PSU (differing ground levels, output voltage variences etc) but in my case both Pi are bieng powered by the same ATX PSU and both on the same 5v rail.
Not yet seen any problems but I'm still planning to power the 1B just via the USB C connection. Need to migrate it to a read only root first.
As always, what works for me may not work for you and "use at your own risk"
The safest approach would be to cut the +ve power line in the cable between the 4B and the USB host.
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