I second Jeff’s advice.
We have done many 16:9 projects with Encore, and had no trouble at all with letterboxed playback on 4:3 sets (Encore does not have a Pan & Scan setting, thankfully).
It can only be either the player is not set up correctly, or the other possibility is the TV set is not picking up the aspect ratio properly. I have lost count of the number of sets I have seen left on factory default, which in 99 cases out of 100 will “assume” just 16:9 playback, even for 4:3 material – making everyone look just like Danny DeVito.
The TV should be set to Automatic.