Cardiff Central no longer has a platform 5, so it's actually just 3 and 4 on the island platform although 5 is still present on old signage.
Personally, I think they should renumber the platforms at Cardiff renaming Platform 0 for Ebbw Vale Parkway Platform 1, the current Platform 1 would become Platform 2 and so on. The current abundance of a platform 5 means the valley lines will still be platforms 6 and 7 (and 8 when the new platform is built).
My understanding is that the tiled signage forms part of the listing of the building, creating the current scenario. As the tiles are mentioned in the listing, its highly unlikely you'd get permission to make any change, hence its stuck with a platform 0.