0w-16 has been around for many years, you can get that in the 5 quart jug. But if you can't find 0w16, you can use the 0w20 without much issue. They are very similar
0w-16 is easily available at your Toyota dealership or even some Walmarts. Do yourself a favor and check the owners manual for your vehicle concerning using any other oil. For example, for my’22 if you use 0W-20, you have to change it within 6months/5Kmiles back to 0W-16. 0W-16 is the only oil that meets 6B requirements for a ‘22 so check your manual for yourself to see if it’s required for your ‘21 (oil that meets 6B requirements, because 0W-16 is the only oil that meets that spec). Failure to use the correct oil grade/specs can and will void your warranty regardless of what Prius Chat “experts”, wannabe “engineers” say. Additionally, there actually is a difference between the 0W-20 and the 0W-16 in that the 0W-20 meets 6A specifications and 0W-16 meets 6B specifications (per AIP and PQIA (the REAL engineers and experts when it comes to lubricants)).
FWIW: Penzoil makes a 0W-16, as does Mobile 1, (which also makes Toyota’s oil, but to Toyota’s specifications (ie: higher levels of additives, per tests that have been published on this site and others)).
BTW, to answer the question of the title of your post: No, not ALL Gen 4’s require 0W-16, just the later years.
