You can request a owner's manual for free at the S&W web site.
250 or so
This gun was manufactured in 1991, according to Smith and Wesson headquarters.
100-460 or so
Take it to a gunsmith
100-425 USD or so
I know the 4006 started production in 1990 and still is today. That ser# I don't know.
Not a user-serviceable part. You need to see a gunsmith.
500 give or take
You can request a manual for free at S&W's web site.
@1990. You will have to call S&W to narrow it down further
Up to roughly $550 +/- in excellent condition used. +/- $450 with average wear.
Go to the S&W web site.