Dr Motor, you can participate in any of the Van Kanighem Tournament stages, starting with whichever one you choose; it's not necessary to participate in all previous ones. The Van Kanighem Tournament is held annually and lasts for almost a year. Stage 1 of VK is dedicated to Heroes M&M 1, stage 2 – HoMM II, stage 3 – HoMM III, stage 4 – HoMM IV, and finally, stage 5 – HoMM V. Starting from stage 6, everything repeats: stage 6 is HoMM I, and so on. You can read more here: this thread.
Why specifically on Whirlwinds? Well, if you’ve read that thread that Lars linked to, you probably understand that Equi has many opponents. And there’s also this point: even those who don’t play HoMM IV normally participate in VK, so they have no reason to study the specifics of Equi...

I would be glad to see you at stage 9 of VK ;)