I've put together some notes about the differences between channel 4 on the T4S and the T1.
The T4S channel 4 compares favourably to the T1 channel 4/5 because T4S channel 4 is fully functional like channels 1, 2, and 3.
T1 channel 4/5 has one function that is not available on the T4S or T8S. T1 channel 4/5 can accept two inputs (left and right from a mixer or instrument) and convert it to mono. You can combine two inputs to mono on the T4S with the Aux inputs (channels 5/6) but there are no physical controls for those channels.
T4S has the Aux inputs (channels 5/6) and there is no equivalent to those channels on the T1.
|T4S ToneMatch Mixer||T1 ToneMatch Audio Engine|
|Channel 4 ||Channel 4/5 |
|Aux Inputs Channels 5/6 ||No equivalent|
You can find more notes about the differences here: