"Jumping the right side of fuse 4 to the left side of fuse 6 did nothing - no pump buzz".

If fuse #4 is defective or blown, it would prevent the main fuel pump from running with it hooked up like this.

The power into the fuse block comes from the battery (+), through the 25A in line fuse, and onto the left side of Fuse terminal #6. It's a big red wire. The primary contact on the fuel relay is powered (continuously) through this fuse

The left side of fuse terminal #4 has two Yellow/red wires attached.
One of these wires is the 12V (+) power feed from the fuel pump relay, and the other is the power feed from the fuel relay (secondary contact) to the main fuel pump. These two wires are not fused at this point, it is merely a junction point.

When the fuel relay closes, it supplies 12V power to the main fuel pump, and power to the in-tank fuel pump through fuse #4. So if fuse #4 is blown, the in-tank pump won't run.

To jumper the pumps, connect the jumper between the left side of fuse terminals #4 and #6. The main pump should run continuously with this jumper installed. You can hear it under the car in front of the rear wheels, to the center.

The in-tank pump will also run as long as this jumper installed.....and....as long as fuse #4 is good. You should hear the in-tank pump if you listen through the fuel filler hole.....use a short piece of hose to help your hearing.

Hope this helps

steve