The ESX-4CV is an expansion module for the ES-4 or ES-5, adding four CV outputs from one expansion header.
In order to squeeze four 12 bit channels through one 8 bit connection, the channels are time-multiplexed and so effectively run at one-eighth of the audio sample rate. For example, if the audio interface/DAW is running at 48kHz, the ESX-4CV's outputs are running at 6kHz. This is still plenty fast enough for most CV purposes, except for audio-rate modulation and the very fastest envelopes. Because the outputs are still derived from the audio stream, they retain the advantages of the ES-3/4 over traditional MIDI-based interfaces, that is, rock solid timing and absence of jitter.
The outputs are hardware calibrated and so do not require the use of e.g. Silent Way's software calibration when used with standard V/octave synths/oscillators. That said, the ESX-4CV's 12 bit resolution is adequate to perform software calibration if required (e.g. to cater for misbehaving oscillators that don't track well to a standard calibration), or to support oscillators with a Hz/V response.