I'm in the same boat. Replaced the cracked screen on my SP4 and shredded the antenna getting the old screen off. Basically nothing left on either side of the top of the antenna bar - just the contacts on the bottom. I've been using the SP4 with the new screen taped in place, hoping I can find an antenna before sealing it up. WiFi and Bluetooth both seem to be working fine; I don't know how to check signal strength...
Main lesson I've learned is that I wish I had bought the MSoft warranty with the SP4. Having lived with the SP4 for only a couple months, I feel that the warranty is a pretty good value.