Can I salvage a missing screw on an iPhone?

If you mean you're missing the screw in the center of the display cable bracket, sorry to say I doubt that's the source of your problems. In the past I've often put iPhones back together for testing and left that entire bracket off, and everything works fine. (The bracket, as best I can tell, is only there to secure the display cable pop connectors from popping off.) If you aren't convinced, you can try moving either of the two lower screws from that bracket into the center position, as they're the same 1.2 mm length. (Do NOT use a longer screw from the upper area.)

That aside, to answer your question, I don't know about salvaging screws from an iPhone 4—it might be possible, but it would be very easy to accidentally cross-thread the screw if it wasn't a perfect fit. Worse, if you use a screw that's a little too long, you can permanently damage the logic board.

If you're determined to find exactly the right screw, you can purchase a replacement screw set. Finding the right screw can still be tricky though, as they come all mixed together. You can use a digital caliper to check the length, and then visually compare a candidate screw with the other screws on that bracket to make sure it's the right size. Thread it carefully and don't overtighten.

Good luck with your repair.