15 feet? I've never heard of anything like this before. Two questions:
What kind of wi-fi range were you able to get prior to your screen replacement? Also, where did you get your iPod touch 4th gen's screen replaced?
Have you tried resetting your router?
Maybe didn't connect the antenna?
I repair iPods, and the 4th gen models are particularly tricky.
It's possible that the antenna (located into the upper left corner of the screen) had been disconnected when the tech was fidgeting the board and digitizer cable to slip it in between the very tiny crack.
Shoot them a message regarding your issue, it's doubtful they'll honor it, but it's worth a try.