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