To check the firmware version on an Olympus DS-4000 digital voice recorder, follow the steps below:
1. Turn OFF the recorder and remove the xD card.
2. Turn the recorder ON.
3. Press and hold the A and B buttons simultaneously for a few seconds until the recorder flashes the firmware version on the LCD.
In my experience, you may have to repeat step 3 a couple of times.
As of 1.6.11, the latest firmware version is 1.15.
Verification Source: In-house testing