As you mention its plugs 4 & 6 - check out this thread and in particular the last 3 posts on the first page
http://www.350z-uk.com/topic/66607-changed-spark-plugs-but-something-not-right/page__hl__+coil%20+packs#entry989242
It is an easy mistake to make, if the leads have been taken off, and I'm just wondering if you might have got caught out