Hello drwavenstein and welcome to the forum.
Can you explain a bit more about how you are using ST4? At first sight, the answer would be "Stop pressing the sustain pedal", but that cannot be right.
All seems OK here:
I have two instruments loaded, triggered by MIDI on different channels. A synth is triggered by the red notes and Sustain is on for the first of those notes. The bass is triggered by the green notes and does not sustain, unless I add a Sustain CC event on its own MIDI channel.