So its a yes for option 2 and 4 from me.
#2 still effects the flow or context of a post, discussion, etc., forcing it clickable just adds a step each reader would need to do to see the image, which is a bit annoying and defeats the purpose of an embedded image, which in a lot of cases can provide valuable information to a post or discussion as you scroll through. #2 is not a horrible option though, as it may take more accountability off the forum itself because the user has to initiate a click to expose their IP to imgur, instead of the forum auto-allowing it through an exception, IF this is a concern. If it is not a concern, #4 seems to be a better option for the forum and its users.