you cant go entirely by what the plug looks like, you need to know the size of the wire its hooked up to, and also very importantly the size of the fuse its on.
assumign, though, that its a 20A circuit, the most you will be able to get out of it is about 4500 watts at 240v. that is 18.75 amps.
if its 220v, the max heater you can use is 4000w (18.2A). so it matters the exact voltage, and the fuse rating behind the plug.