The ep5.4 game.exe is not compatible with the ep4.5 server files. The last game.exe that works well with the ep4.5 server files is the ep5.3 one, which most private servers are using.
As for the client as a whole, apart from the game.exe, there's really nothing to worry about, except maybe the ep6 .SData files which have some new properties for whatever it is they define (Skills, Items, ...). There's also a new format for the .3DC files, but castor released a converter for that, which gives you the ability to convert new format objects into old format so your game.exe can read them.