Pics of your gun would be very helpful for us to give you a fair guess on what it would sell for.
Also, specify whether you have the box and tools that the pistol was sold in when new, as that also adds to the value of the gun.
And the guys above are right about them being more scarce than the 3 1/2" barrel. The 4" -2 model was only made from 1979 to 1981 after S&W discontinued the 3 1/2 and 5" models.
As to the rear sight blade, you can replace that with a new sight blade easily and cheaply. S&W has them, as well as Brownells and Midway.
As to value, one in very good condition and has the box/tools would easily sell for over $1000 on Gunbroker.