Seems like Suda can't do much with the IP (like a Shinobu game) without Marvelous' consent as they only own 10% of the IP. NMH4 would not be before a decade according to him as he wants to focus on new IPs for the company and he wants to give the series a rest.
NMH3 is 15-20 hours if you play the game and do some of the side content.
Apparently previews are coming out on monday