To play the notes of the Jupiter-4, an internal retrofit is really the only option.
The Kenton MIDI retrofit kit for this model is aimed at controlling the Jupiter-4 using MIDI. In other words MIDI IN only.

For incoming MIDI:
Any MIDI channel can be selected
Notes (MIDI note numbers 36 - 96) - transpose possible
Program change (numbers 1 - 40)
Pitchbend - (bends up and down a fifth)
Mod Wheel
Filter cutoff frequency (of low pass filter)
Volume
Sustain (decay on-off)
Arpeggio clock synced to MIDI clock - variable ratio allowing 1/4 1/8 & 1/16th notes as well as triplets.

Please click here to see the retrofit info pages for more details about the retrofit kit.

Clock the Arpeggiator

If you don't need to play the notes using MIDI, you can clock the arpeggiator using the Pro-SOLO mkII or a Pro-2000 or even a Pro-KADI, as they all have clock outputs. You will need a 3.5mm to 6.35mm jack cable.

click here for 3.5mm to 6.35mm jack cables (1 required):

You can also control the filter cutoff frequency (VCF) using a Pro-SOLO mkII or a Pro-2000 mkII. Again you will need a 3.5mm to 6.35mm jack cable.