"No antenna for wifi". This is true for iPhone 4, but iPhone 4S does use a separate wifi antenna, not the top bezel. Top screw simply holds the logic board onto the bezel to ensure cellular and GPS signal goes through. so I conclude Gido's answer not helping
The iPhone 4S has a wifi antenna around the headphone jack, it is easy to replace. Open up the device, look for 2 screws beside the headphone jack, one holds the antenna connector bracket, one grounds the antenna, remove them, disconnect the connector and peel the antenna off.
Ifixit has a way too over-disassembled guide calls it "upper antenna". That's the wifi antenna. You can use the guide for reference but don't follow the guide to remove everything