I am assuming the four screws to which you are referring to are on the back of the petcock, #221 in the attached illustration. They are holding the outer plate of the vacuum fuel shutoff valve and are 4mm Philips head screws usually needing a #2 Philips screwdriver. You will also need a new vacuum diaphragm, #43028 in the illustration. 1086 Vulcan petcock exploded view.jpg
It tends to get brittle with continuous exposure to fuel and can crack, sending raw fuel into the cylinders. This is bad as it can wash off the oil necessary for the sealing and lubrication of the piston rings, causing scoring of the cylinder walls necessitating a top end rebuild with all the attendant parts and labor bills. Of course it also can drop gas onto the motor, which with little provocation can ignite and leave you standing by the side of the road watching your pride and joy burn to the ground upon which it is laying. Trust me, it is a very bad feeling!
Obviously I recommend taking care of this small problem before it becomes a big one.
If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------