Unfortunatlely there is no AM filter for the TR4C, at least Inrad doesn't make one. That rig uses seperate filters for Upper and Lower sideband and in AM mode it goes to LSB. The filter bandwidth is 2500 HZ so it will not sound too good. Like Steve said, you can tune to the low side of the filter to get more high frequency but it was never really intended as an AM rig. I had the same problem with my TR4CW and the only AM I ever listened to on it was WWV.
If it weren't for Philo T. Farnsworth we'd still be eating frozen radio dinners.