Just got a MT-4 second hand and it came with a lipo. The battery has two receiver style connectors coming off of it. Can I charge it with either one? Do I need some kind of a Y adapter to charge it? Only one of them is plugged into the transmitter.

This is the battery:
http://www.hobbypeople.net/index.php...-2300-20c.html

I put a voltmeter to both connectors and they read the same. I assume that means I can use either one for charging or for power to my transmitter?

You can charge from either, or plug in either. The dual plug is so that you can leave it in transmitter while charging (connect to balance connector and other plug)