pulling cells from a 4th gen battery

They'll work fine as long as they're NiMH. The Gen 4 NiMH module is dimensionally identical to the Gen 2 module.
The Lithium batteries are dimensionally different and will not work in the Gen2 case.

 

they come and go like everything these days

 

I wouldn't bother even looking for the OEM batteries, you can get the cylindrical packs for much cheaper. Used batteries can be hit or miss... $1000 gamble, but with that low of mileage should be ok. How long have they been sitting and in what condition?

 

Sure:

• NewPriusBatteries
  
• https://electronhybridsolution.com/product/toyota-prius-2010-2015-brand-new-hybrid-battery-rebuild-kit/

Probably a few more places beyond those. ChrisFix posted a step by step video on switching them out here

.

 

Ummm. there are tons of Gen 4 NiMH batteries available from low mile late model wrecks of Prius, Camry, Lexus, etc vehicles. Although most very late model ones are in the 1k range, they can be had for as low as $500-600 if you keep your eyes open. I would take a set of modules from a late model Gen 4 over a cylindrical any day of the week.

 

Whichever you choose, do let us know how it went and how the MPG is after the swap. Thanks!

 

Gen 4 batteries G9510-47130 NIMH 2016-2022

2020-2022 Corolla NIMH hybrid are also a good bet in salvage yards.

 

Cylindrical packs? Have you had personal experience with a cylindrical pack? Not sure why you are so gungho with no personal experience other than watching a ChrisFix video. I'm more inclined to take @TMR-JWAP suggestions before yours.

Prius Battery Replacement Kit (GenII/GenIII) with NEW custom cells | Page 46 | PriusChat
Prius Battery Replacement Kit (GenII/GenIII) with NEW custom cells | Page 47 | PriusChat