There is confusion. In the U.S When something is given a numerical addition, it indicates a completely new airframe (or "weapons system" as the military likes to call them). The F-35 (Lightning II) is in no way shape or form an advanced version of the P-38 (Lightning)!!
Now the U.S has screwed this up with the F-15ex "Eagle II", which is still just a development of the F-15 "weapons system".
The Dralthi 4 is a "correct" designation; since the WC3 version seems to be a new "weapons system". As for the Arrow and Thunderbolt in WC3, when calling them "5" and "7" respectively, does this mean they are the 5th and 7th variant of the same "weapons system"? Who knows.