With part 4 I thought I run down the main suspects (somewhat in order):

The main things to look at: Do they have the motivation? Do they have
the capability to do it (time, power, other)? Can they get a girl to sit
through what victem four went trough.

Mousse: Showing signs he know Shampoo would never return his love, and
he might be taking that frustration out on all girls. Questionable
placement at night. Likely to know paralyzation pressure points or has
'borrowed' something from Cologne.

Konatsu: Screwed up childhood. Most of his problems are girl related.
Out at nights 'searching for the killer.' Know to use paralyzation
powder.

Saotome Ranma: This boy has the motivation to hate girls a plenty
between his curse and multiple fiancees. Plus he has gone out at nights
returns as a cat. Likely to know paralyzation pressure points.

Tsubasa: Rejected by his family for his unconvenition pursuit of love
and is rejected by the one he wants the most. Problem with him is how
does he get a girl to allow him to finger them and not be restrained.
(Question: Since when is Tsubasa a constant annoyance to the Ranma cast?
He appears once in either the manga and anime with two more cameo
appearances in anime.)

Happosai: He does what he wants, might hate women enough. Also, likely
to know paralyzation pressure points.

Hibiki Ryoga: While he has the power, and the potential motivation- his
directionless sense curse doesn't make it likely he could be the killer.
Potential to know paralyzation pressure points.

Gosunguki: Has the potential motivation but unless he's found some way
to beef up and restrain a girl without ropes, he can't be the killer.

Unnamed possibilities:

Mikado: He does live in the area and his kiss a hundred girls may turn
into more. His charm would get the girls into that position and his
strength can do wonders on someone's face.

Bake-Neko: Where do you think all those cats are coming from? He's been
possessing various people and is trying to find a bride, but they keep
rejecting him. One of the few forces that can consistantly send Ranma
into Nekoken and Ranma's phobia could cause him to forget encounters
with the ghost cat. (This is who I think the killer is ;)


Most of the cast are still possible with the exception of Akane, unless
she's developed a secondary personality to handle the killing because
we've seen her thoughts and those don't match up if she did it.