Definitely don't do one if you did an away there. If you couldn't see what you needed to in 4 weeks, another day or two don't change anything.
Also, most (>95%) programs do not care if you come back for a second look. Second looks should be for YOU to decide between your top couple programs. There are however a few programs that pretty much require it to be ranked and they will make that clear to you on interview day and the pre-interview dinner.