On the SMARTDAC+ GX20 recorder, can I record on channel 4 every 1 ms?

Yes, with 4 high speed universal analog input modules and the GX20-2 (large memory model).
* At a scan interval of 1 ms, one module of the high speed universal measures 1 channel. (Scan interval 2 ms: 2 channels/module, scan interval 5 ms or more: 4 channels/module.)
*See page 21 of the specification (limit on the number of measurement channels that can be used on the GX/GP)