I wouldn't call them rare, but they are desirable.
Many consider the 686-4 to be the best iterations produced. They still have a forged trigger and hammer with a hammer nose, use the old style cylinder stop stud, the frames are drilled and tapped for optics mounting and they feature the new style rear sight with a rounded front.
The 686-4 was definitely made in both standard (6 shot) and Plus (7 shot) models. I have one standard 686-4 4" and two of the 7 shot versions, a 2-1/2" Magnaported Lew Horton "Limited Edition" and a 6". The cylinder stop notches and offset from the chambers on the Plus version, so the cylinders are theoretically stronger.
The standard -4's came out in 1993 and were replaced by the -5 in 1997, but the Plus version of the -4 wasn't introduced until 1995. I have no idea on quantities, but they were built for a shorter period of time.
Edit to add:
There are a couple of downsides that you should be aware of.
Formed/boned holsters are different for 6 and 7 shot cylinders, because of the flutes. Some will interchange just fine, but others like the Kramers are stiff don't easily accept the wrong gun. If you have both versions of the gun with the same barrel length, order a holster for an unfluted cylinder.
Another challenge is counting your shots and knowing when your gun is empty. I'm accustomed to shooting 5 and 6 shot revolvers and don't have to think about round count, but I struggle when shooting a 7 or 8 shot revolver. Old habits are hard to break!