Death Knight scourge strike

I have a question, you see, frost strike and heart strike are both trainable till rank 6, why is Scourge Strike the only ability that is trainable till rank 4. Is this the way it's supposed to be? Was it always like that even it retail back in the wotlk days?

Yes, Scourge Strike only goes to 4.

So it's not just a Warmane thing? Is it like this in other private servers?

It was like this in retail also. It is just the way it is.