[EricH]

Andrew, as I recall the pre-'05 rules, not only did every team have to play at least once, but every team had to play at least once in each round of elims. That is:

QFs, if you played A-B in your first match, either A or B had to sit out your second match so C could play.
SFs, if you played A-B in your first match, either A or B had to sit out your second match so C could play.
Finals, if you played A-B in your first match, either A or B had to sit out your second match so C could play.

Third matches were up for grabs.

If there was such a rule about "4th robots have to play", I would expect to see something like this rule, as it disadvantages both alliances relatively evenly.