"824tsv: As we've all come to find, & know, S&W's relationship between the SN# & Spec Order# (ship date) is a twisted one. I just looked at my 4006's box label & your's was made 36 days after mine but it has a SN# 6400 before mine."

As we have come to learn:
The first rule of thumb with S&W is there are no rules.......

As a supplement to my previous post with this thread, I took the 4006 out to the range this afternoon and had a smile on my face the whole time shooting it. It's a wonderful pistol. If the OP ends up with the one he is looking at, I would assume he will have the same smile as mine.