You're correct in that. If one member posts 4 pages from one issue, another member may post 3 additional pages from that same issue, for a total of 7 pages.
In the other Dark Avengers #5 post we appear to be over the 4 page per poster limit, and I've left a note asking for some of the scans to be taken down. That being said, if that first post leaves 4 pages up, then you would be limited to posting 3 pages.