The box shouldn't be freezing and rebooting every 4 hours, however check the standby timer settings, and amend them if necessary to see if this helps prevent the box shutting down.
Settings > System > Standby Timer
There are 4 options. 4 hours, 5 hours, 6 hours and 24 hours.
I have mine set to 24 hours. If I left it set on any of the other options, for example 4 hours, then the box would shut down after 4 hours.
You could try a factory reset.
To factory reset go to Settings (the cog at the end of the Home menu) > System > Factory Reset. It's actually misnamed and only a partial system reset and not a complete factory reset. You are given 2 options
Keep Recordings
Format Disk
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up. You will have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again. Also Match frame rate and Standby power consumption will go back to their default settings if you have changed them, so I would double check Standy power and set it to 24 hours.