Cylinder number 4 and fuel bank 1 locations?

You are right that cylinder 4 is closest to the driver's side.

DTC P0172 means that the air/fuel ratio is too rich. The reference to bank 1, sensor 1 simply means that the issue can be with the upstream air/fuel ratio sensor. There is only one bank. Some cars have dual exhaust and such vehicles would have a bank 1 and a bank 2.

What did the old spark plug in cylinder 4 look like and how did it compare to the other three plugs?

If cylinder 4 showed a very dark plug, that would be an indication of an overly rich fuel mixture and the fuel injector would be a reasonable candidate to suspect.

 

I think that the misfire can cause rich running code. By letting unburned fuel into exhaust. So fault may also be with ignition system. Easiest check would be to swap ignition coil from cylinder 4 with ignition coil of another cylinder. If misfire moves with coil then coil is faulty. If it doesn’t then problem can be with fuel system, rest of ignition system, or mechanical.

 

Prius engine has only one bank. Bank1 all 4 cylinders belong to that one bank.

Normally inline four cylinder engines have only one bank. And in rare cases they have two its cylinders 1 and 4 in bank1 and cylinders 2 and 3 in bank2

So rich code did come back but misfire didn’t? Did all old plugs look the same?

 

Your question was answered in my post #2 above. Yes, cylinder 4 is at the "rear" of the engine and is closest to the driver's side.

 

Jason, any updates? Did you check or change the injector at cylinder #4?