The software has identified four users as having suspicious voting patterns. Three of these users share an IP address with the user they're voting for. However, as far as I can tell, none of them are sockpuppets; my best guess is that they are acquaintances sharing an IP address for one reason or another. But I still do not approve of this voting behavior. Is this worth contacting those users over?
This is probably worth contacting about.
Whether or not deduct some reputation or some other action? This should be based on the amount of reputation earned and your (perhaps best to confer the other moderators on the topic as well) judgment and the reaction from the users (i.e. whether or not they understand that this is something not to be repeated)
Edit: Also, is it the same IP or the same IP range, and is the IP a proxy server (e.g. a university IP address)
And I think it is best to add something about that in regards to "voting" requesting users to vote by their confidence of the content and not the person.