Rank 0,1,2,3,4

Okay, I was looking on the Halo wars stats page, and it has unit ranks, 0,1,2,3 and 4
Rank 0 is likely a regular unit, rank 1, a 1 star, and so on.
But what is a rank 4 unit then, because I though that the highest veterancy a unit could get was 3 stars?

I think ranks define how many units a unit has killed, rather than being a part of the veterancy (1-3) system…

Correct me if I’m wrong.

Hmm its a nice theory, but I don’t think thats it.
Apparently the rank 4 unit is a legendary, that is no one kn ows how to get it exactly…

Any evidence to suggest that?
Possibly a rank 4 unit is one that contains a spartan (unit with 3 stars)?

Wraiths in DM can get 15 stars -> Wraith in DM = Rank 4