If you search for PC threads with Prism, or DadDadDad posting, you'll find quite a few. (Around 2013-2015)
https://www.mumsnet.com/talk/pedants_corner/1778694-Less-than-or-fewer-than
It's to do with discrete and non-discrete use.
A year is countable.
But we don't say "fewer than 4 years", we say "less than 4 years" because we are considering (semantically and pragmatically) not a "year", but "time".
Individual objects in a pile of shopping are countable. Boxes of cereal, tins of soup. Shopping isn't. So "5 items or fewer" is correct. (If semantically you consider the individual items and not what the individual items are a part of) But "5 items or less" isn't wrong (if you consider the individual items as part of the whole)
It's really only at a very simplistic level that countable= fewer, uncountable= less. Corpus studies' findings also indicate that "fewer" will soon (in linguistic terms) be obsolete. Won't be in our lifetime, obviously.