It only shuts off after 4 hours if it doesn't receive any input from the remote. The 4 hour timer resets every time you press the remote. The chances of anyone watching TV and not using the remote in a 4 hour period is small.
It's a feature to save energy by turning off the display output. It's not the power on the PVR that it's saving, it's the power consumed by the TV. Most TV's will go into standby mode after a while if it senses there is no input signal. This is useful if you fall asleep watching TV or if you have kids that tend to turn on the TV and then go play somewhere else.
This has no impact on the PVR's ability to record shows as the previous posters already mentioned.
I'd like to add, you can have the Standby feature extended to turn off after 24 hours instead of 4. I know some people for example, who like to have it on for radio purposes or their pets at home. This feature however, as mentioned above, does not interrupt the PVR's capabilities whatsoever.