I recently saw this thread, which contains specifications for the pulse durations. Specifically, it appears that a 0.02 second dropout indicates AV synchronization and a 0.07 second dropout indicates audible silence synchronization. Could there be some value in the middle (such as 0.05 seconds) that causes T4 synchronization? There is also a 0.02 second on-pulse at the beginning that starts synchronization, but changing this value or sending the pulse twice might cause the horn to output T4. The fact that the strobe starts before the horn may be a sign that the latter mechanism is used.
If this can be legally achieved, it may be worth experimenting with this to figure out how to synchronize these horns on T4. It would be interesting if the fire alarm community activated this functionality before System Sensor officially released it.