b450m pro4 PSU cable question

No, don't use an adapter on the 6pin, that is for PCIe power not CPU power. You
can use a single 4 pin occupying one side of the motherboard connector. You can
use a 4 to 8 pin adapter too but, get a decent power supply. If it doesn't have
an 8pin CPU power connector it is either a cheap piece of junk or old. Just because
"it will work" doesn't mean you should use it. Older power supplies that don't
have an 8 pin tend to not be up to the task of running newer boards, if the PSU
is old enough it may not even have enough power on the 12v rail for the system
to run stably. Cheap junk PSUs are often leftover new old stock that is repackaged
and sold cheap, often with misleading labeling like Max 450w when the actual wattage
is 250w for example.

Do yourself a favor and use a decent PSU, it should at least be 80+ certified.
A bad PSU can damage every component in your system, it is the last thing you
want to cheap out on. Take it from someone who fell for the whole "but this PSU
worked fine in my old PC" only to have it pop and kill my motherboard, hard drives,
GPU and CPU. An expensive mistake all because I thought I could get away with
not upgrading it and put the money toward a better GPU....

These days, I won't even use a PSU like that for testing purposes.