I don't know if they reclassified it at some point, but back on those days 3.5 was titled "Windows for Workgroups" and 4.0 was the first to be known simply as "NT".
Forget what I said, I recalled an old memory from childhood of a 3.5 upgrade box for people running Windows for Workgroups.
NT 4.0 is definitely what popularized that version prior to Windows 2000 and XP. Most people who just say "Windows NT" are thinking about 4.0.
Replace NT in this list with ME and you have all the consumer versions. NT versions 3.5 and 4 were the business versions in parallel with 95, 98, and ME.
Win2k wasn't consumer. It was the business offering at the same time as ME, which may be surprising to some. Xp was their successor, merging the business and personal lines.
Yeah, those were the days, back when more often than not a Windows upgrade was also an improvement. As much as I loved Win2k, WinXP was even better. Let's not talk about Vista and while Win7 was nice, it wasn't much of a UX improvement.