I had a previous beta installed.
When I did the bootloader 17L patch, it caused the unit to hang on the 'logo' screen after saying it'd successfully installed the bootloader.
Called technical support in the Netherlands (very helpful!) who suggested I install the V4 beta 5 patch, which apparently also includes the bootloader, but might more successfully install it. This worked fine, (and I noticed that the beta 5 version didn't have the problem I was experiencing.)
Then installed V4 firmware again, this time no problem with the unit rebooting when I play media files, so (hopefully) all is now OK...
Now to work out how to get all these new features to work!