Hi,
in short yes.
The cable with four wires, usually thick gauge wires, are the three phases and one earth wire for the motor. The cable with 15 wires is almost certainly
the encoder and hall sensors. The probability that the servo drive is matched to the replacement servo is vanishingly small.
I would recommend, especially if you are new to servos, buy a matched pair.
I have found Delta, a Taiwanese brand, manufactured in China to be good quality, good backup, good documentation and MOST importantly
free PC based tuning and set-up software, at very fair prices. There are even cheaper brands, all Chinese made, but have questionable backup and
support, poor documentation and NO set-up and tuning software. DO NOT BE TEMPTED.....you will tear your hair out with these things if you are new to servos.
If you've had plenty of experience with AC servos THEN they are very attractive as they are cheap and perform well and are a good proposition.
Just as a matter of interest the Delta servos only have four wires in the encoder cable as the encoder/servo drive communication is highspeed serial.
That all but ensures that if you have a Delta servo you MUST use a Delta servo drive in order both ends of the serial link match each other.
Craig