I think you mean part number 3, the gas regulator as illustrated here:
http://www.e-gunparts.com/productschem.asp?chrMasterModel=1930z40
Part number 4 is a wedge that holds on the whole front sight / muzzle brake assembly. I never tried to remove that on my rifle.
To remove the regulator, take off the handguard, remove part 6, the gas piston by pulling back on part 7, push its end to the side and pull it forward to remove it, now you can pull the piston off to the rear. Use a small wrench to unscrew part 5, the gas cylinder and remove it. Now you can remove the regulator, part 3 to the rear, if it sticks tap on the front end (it is pentagonal in cross section and has numbers stamped on it) that sticks out thru part 2. Something else to look for is loosness between the gas piston and gas cylinder, if there is it will allow gas to blow by reducing the thrust of the gas piston.
The attached picture is of a Finnish combo tool (SA and VKT marked) I have for the SVT40, it came with my rifle, I also have a front sight adjustment tool, and I expect that these are not too common, I have never seen another. The key-hole opening is to remove the end of the muzzle break to clean it, the hole in the end is pentagon shaped to adjust the gas regulator and the large notch is to loosen the gas cylinder. I forget what the other notch is for; the penny is to gauge size.
