RE: Can I use a present servo in old system radio
The forth wire in a four wire servo was for the center tap of the battery required in the old servos. It is not required for current servos. New servos should work if the pulse is of the right polarity. Some of the older systems pulse polarity opposite of current standard.
Make sure the +, - match, and the signal wire match on the connector and try it. Ignore the forth wire. You can't hurt it if the + and - wires are correct