A player of four days is not a base player. He is a newbie. A base player is someone who is aware of the existence of other mods and has played them, then chosen base over them. If a base player of three months plays against a Shifter player of three months, then the base player will win hands down.
And if a base player goes to a mod and gets owned, then he was getting owned back in base, as well. Anyone who does well in base owns every mod.
Furthermore, TribesRPG isn't a mod, so I don't know what the hell you're talking about. TribesRPG is a Total Conversion.