From the Alpha Demo of 5, he's actually been nerfed. Not immensely, but enough that it's more difficult to spam with him, and Izuna-experts have to jump through the extra input commands of his new counter-system.
I assume by "Ninpo stance" you're referring to the Ongyoin, and that replacing his handstand as a primary stance is really what boosted him up to top tier in 4 (though his speed seems increased too, but maybe I'm mistaken).
Hayate, imo, is still even more over-powered with higher damage and a list of aerial moves that seem to go unmatched. But, I seem to be mostly alone in that, as most consider Ryu the best fighter in 4.
I just call it ninpo(as do others)cause he says nin and in ninja gaiden blahblahetcetc. Yea I think were on the same page about that. Even still DOA4 was kinda fun even still. I loved the Hayate vs Ryu matchup but I just always felt at a disadvantage.I didn't think he was all that strong in DOA4, he wasn't bottom tier obviously.
I mean he was kind of a cross between Ein and Kasumi in a sense. He gains a bit more speed and mobility type moves while keeping some of his crushing blows and iconic launchers(like Fuijin(236p) 214k, 8p, 8k), and gaining some newer moves that show the signs of being a true user of the mugen teshin style(7k, 4kk, and even 66kkk which he and ayane share, as well as her version of 7k)
In all honesty, I'd love to see him pull off kasumi's 6pkk, I love that string with a passion.
When I honestly think about it, If I tried to match up some moves side by side from Hayabusa and Hayate, I'd probably come to the conclusion that they both have similar tools at their disposal... which I might actually do right now. Could be fun.