I remember playing Commander Keen 4 demo a lot (I finished all the demo levels) then searching for a free version full download in google. (I used to do that a lot when I was younger.) somehow they never yielded any free versions or downloads.

The game was a blast. Then I played and completed Keen 1 when it was released as freeware. They felt like completely different games (Keen 4 is a lot bigger game than Keen 1 which is like Duke 1 compared to Nukem 2).

I still have all 6 of them (Older me now knows how to get full versions without the stupid google search terms) but I no longer play them though they still as good as they used to be.