Your setting are wrong, if your viewers are 16:9 your sequence is 16:9.
If you are using FCP7 create a new sequence, drag a clip into the empty timeline, it will ask if you want to change the settings to match, click yes. If the clip is 4:3 the sequence will be 4:3.
NOTE: changing settings like Easy Setup in FCP are for any NEW sequences, they will not change any that have already been created.