Well...I think the Onimusha series are ok as they are now: ENDED and CLOSED.
By this I mean the following: Onimusha 1 was a short game but incredibly good. Very polished, with cool graphics (they were nice despite prerrended), an awesome soundtrack (I think it's one of the best 10 soundtracks ever) and an ambience that was very nice and beliable. The japanese castle is so cool and well represented that I loved playing the game.
Then Onimusha 2 introduced some things that were very good like the gifts a 10000x times better main character (Yusaku Matsuda >>>> Takeshi Kaneshiro; sorry Takeshi, I liked you a lot in Fallen Angels but in Onimusha Yusaku rocks

) but in general terms te story lacked interest and was less beliable. The music was worse imo.
Onimusha 3 was an awesome experience from the gameplay point of view...but story-wise you soon realize that sometimes the guys at CAPCOM drink too much (as Mikami and friend proved in the Wesker Report). This is more agravating if you consider that many Onimusha fans played it because the japanese classic ambience and not because the sci-fi/terror elements. Despite all this the game was pretty good but well...
So instead of seeing an Onimusha 4 being horrible as hell, I prefer to leave things as they are now. I am sure that a 4th game would mean more Sci-Fi and less medieval pacing...And well, I am not interested on this.