Is it permissible for someone who does not intend to shorten the prayer (qasr) to complete his four-rak'ah prayer behind an Imam who intended to shorten it and made the tasleem after two rak'ahs?

The best is to shorten the prayer with the traveling Imam, but for a traveler to pray a complete prayer is permissible. Following someone who is already following an Imam (i.e., a follower) is disputed among scholars; some scholars deem it invalid, while others consider it valid. Sheikh Ibn 'Uthaymeen affirmed the validity of following a follower, but he disliked it because it was not known among the Salaf (early generations of Muslims), so it is better to avoid it. Your prayer behind your colleague is valid, but this should not be repeated because it was not transmitted from the Salaf and the majority of scholars do not validate it.