Welcome to the forum. I don't have a 4006. However, I have changed to adjustable sights on a 5906 and a 4566TSW. I never had a problem with the accuracy of the original sights. I just like the accuracy at longer distances. I did keep the original sights as the gun may hold it's value better in it's original form.
Your 4006 would use the same sights. I obtained Millet sights from Optics Planet. You must replace the front and rear sights. They do not come as a set but must be ordered separately. I do not know if those sights are still available.
Instructions with the new sights were quite good and it was a simple job of drifting out the old sights and drifting in the new. CAUTION, when removing the rear sight there is a spring under it that must be restrained during removal of the sight and must be held in place while inserting the new rear sight. It may sound treacherous but was not as bad as it sounds.
On my 5906 I had to file a small amount off the bottom of the dovetail on the new rear sight to get it to start into the dove tail in the slide. (This was covered in the instructions but filing was not required for the installation on the 4566. Also the dovetails on the slide are tapered. When drifting the old sights out they will only go out in one direction. I do not remember which direction but that information is available, either in the directions with the sights or from a call to S&W.
I am quite happy with the Millet sights as they are quite durable and are no higher that the originals. Good luck and good shooting.