There are two variants of the RM - F04 remote control as used with the HD and HDR T2 units.
To check which variant you have remove the battery cover and look for a white label:
If label states - MDB 1.3 then 3 digit code.
If label states - ADB 1.0 then 4 digit code.
With thanks to a couple of members who have pointed this out, a few remotes were initially issued with ADB 1.0 label that use the 3 digit codes. This has been confirmed by Humax Support here:
http://myhumax.org/forum/topic/remote-control-confused-if-3-or-4-digit-codes#post-326