The four digit "code" isn't the IR commands for your TV. It's the location of stored IR codes in a particular make and model of "universal remote". That "code" will vary from one remote to another for the same TV. If the remote doesn't have the codes for your TV installed at the factory then you can't enter them.
The Logitech Harmony remotes are programmed online from their database of remote codes. If the database doesn't have the codes for a product they can be learned from your original remote so the next time they are needed they are already in the database. They also have at least the same number of functions as the original remote.