It sounds like you may have changed, and forgotten, the remote code on the 4000T.
Is the replacement remote an official Humax replacement or something either generic or from another manufacturer?
If it isn’t Humax you won’t be able to change the code.
If it is a Humax remote follow the instructions at https://myhumax.org/forum/topic/multi-code-function-hdhdr-remote-2. As you can’t get the 4000T to respond press 0 and OK for the 8-10 seconds and assume the remote has gone into the setup mode. Then press 2 and then try channel up/down and see if the 4000T responds. If it doesn’t repeat the whole process with 3 or 4 or 5 or 6 until it works.
If it never works you could try setting 1 which I assumed was the default setting.
If all else fails, or the new remote isn’t
a genuine Humax, and the new remote will operate the 5000T swap the remotes and see if the old remote will work the 4000T. If necessary repeat the above process on the old remote/4000T combination.