Yes, you can just short the pins/pads of J4 with solder or a small piece of wire. But if you ever plan on installing the expression upgrade then use the provided 2-pin header and jumper to short these pins together. When the expression board is used, the jumper on the header needs to be removed because there is a switch on the expression board that handles the tuner output functionality instead of this jumper. All this jumper does is complete the signal path to the tuner jack…if it’s not shorted then no signal gets to the tuner jack, unless you are using the expression upgrade board, which that routing is controlled by a dip switch on it.
I hope that makes sense.
-Brach