At the worst, the xbee will draw too much, the 3.3V regulator will get warm, the power will sag, you'll lose comm's.
More likely - you have 4.2 to 5V on Tx out to the Xbee, possibly causing damage to Xbee inputs that are expecting inputs no greater than VCC + 0.5V = 3.8V.
May take longer term to become apparent, on either the Arduino or the Xbee.