The TV zapping model currently only allows typing a maximum of 4 digits for a channel number, see here https://github.com/MediaPortal/MediaPortal-2/blob/MP2.4.1_Final/MediaPortal/Source/UI/TV/SlimTvClient/Models/ChannelZapModel.cs#L84

Some users are reporting that they have 5 digit numbers. There doesn't seem to be a reason to arbitrarily limit the number of digits beyond ensuring that the UI displays them correctly. This limitation should be removed to allow any number of digits.