This was actually a retail product back in the day... you used to be able to buy this demo disc for $5 at Best Buy. (There was a sleeve which I haven't tracked down yet.)
Several versions of the Wing Commander IV Demo exist. The first included a cinematic trailer and was released on magazine cover discs (here - 43 megs). That was a huge file for an era when ordinary people didn't have CD burners or fast internet connections... so Origin later released a smaller internet version with the trailer removed (here - 14 megs). Even that seemed like an impossible download at the time, though! I remember sending 30-odd hastily-blanked discs to work with my dad in the hopes he could download it for me... but one was always corrupt! A Macintosh version was later released (here - 6 megs).
One secret to the Wing Commander IV demo--you can actually "pick up" a Stormfire to play with! Right after you take off, target Dekker's shuttle and follow it for about ten seconds... it will drop a new gun you can fly into (a la Arena) and then use!