Before you order anything can I please set you straight on what you need?
The hex wrench you require is 5mm not 4mm and the one that Sauer supplies is very nearly 11" long with a 5mm female socket on one end. A separate 90° 5mm hex wrench is inserted into the female end to undo the bolt that holds the forend on. Also in the kit is a 90° 4mm hex wrench which is used to undo the 3 retaining bolts that allow you to remove the barrel from the receiver. I don't believe the one that WapitiBob suggested will fit into your forend even if it was 5mm.
Please see the photo of my Sauer supplied tools. I hope this helps.
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