I'm a fan of the King's Bounty franchise, but I'm a bit behind with it. I've played Legend twice, Princess once and North never.
I liked King's Bounty a lot, obviously, but was/am always frustrated by the fact that Cavalry, Swordsmen and Knights are gated behind artificial progression barriers and then, when you do pass the barrier, there aren't enough of them for what you want to do - I always end up with predominantly Pirate and Snake armies then gradually convert to Elven armies, just because they are more numerous and have less 'compatibility' issues. The compatibility issues are annoying also. These are the only issues I have with the game however, so that's me a pretty happy gamer.
Princess was OK, I got on with it fine, pretty much the same as above apart from the fact that my memory is just over-filled with images of Imps laughing and AoE'ing my troops in every single battle, that got really tiresome - probably due to the Crossworld's expansion more than a fault with the original game. But my main issue with the sequel was that the music didn't seem to be as good as the original. I'm a big fan of music in games and often play games just to hear the tunes they provide. The Elven music in Legend is sooo nice and the Pirates are so comical. So Princess just didn't have such a good feel.
I never got to the North, the third game, so I haven't got that as a potential negative in my mind.
With regards to a fourth, who knows. KB is a game which is able to repeat the formula quite easily and get away with it, in the same way mario or Worms can, but the devil will be in the detail. Will it feel good, or will it feel like a cash-in, if you know what I mean. I felt a lot of love went into Legend and, to an extent, Princess. It sounds like North was more a cash-in, I guess we wont know until it's released.
I'm not looking forward to the whole 'dark' thing though. One of the beauties of Legend was that it bucked the trend by being unoriginal and just giving you a straight good vs evil scenario of knights in shining armour, in an era where everything has to be dark, even Superman (for heaven's sake) it actually makes KB Legend feel like the most original game out there. I LIKE knights in shining armour cliches! I wont be choosing the 'dark' options, so all that means nothing to me!