I currently own a Jupiter 4 and Jupiter 8. I also owned a Jupiter 6.
I bought the JP4 first, and it has only ever blown me away. It is the most unique synth i've ever used. For the type of music I make it is perfect.
The JP8 I own is my second. I bought one and never took to it. I found it kind of light and 80's pop. I wanted The Human League, Yazoo, Gary Numan, and the Jupiter 8 wasn't giving me that, so I sold it.
6 months later I came across a live recording I had made with it and it blew me away. Luckily I had downloaded my patches before selling it, so I tracked down another and got the sounds back.
Basically I hadnt given it enough time. Its a far more complex synth than the JP4 and required more time exploring it to reveal its abilities. Yes, it does 80's pop brilliantly, but it will also do dark, pulsating, post apocalyptic sounds. It is a masterpiece of a synth.
So the answer to your question is unfortunately, buy both if you can. They are two amazing synths, but they are not comparable. The personality of the JP4 for me is far more interesting. I see it as the uglier, but more interesting older brother.
Which would I choose if I had to? I'm really not sure. Maybe the JP8 because it is more versatile, but the JP4 has a more unique tone.
To help you out I would say base your decision on the price. I'm not saying buy the cheapest, but buy the one that represents the best value based on current market prices. If the JP8 is cheaper than normal, buy that one, because they'll just keep going up. But the JP4 is still around the $2000 makr, so you'll have time to get one in the future before they go too mad.
This might help. One of my musical projects, KuBO. The sequence in this track is JP8. Bass is System 100. Strings are Korg Monopoly.
An Oddity by KuBO on SoundCloud - Hear the worldβs sounds
Lead string type sounds in this are JP8. Portamento sound is bridge is also JP8, as it string on chorus. Bass is System 100. Riff is Prodigy.
This Desolation by KuBO on SoundCloud - Hear the worldβs sounds
Much of this is JP8. All of the sounds at the start. The bass line, the drone in the chorus. Lots of JP8.
Come To My Disco by KuBO on SoundCloud - Hear the worldβs sounds
This is my other Musical project, PolyDROID. The whole first min of this track is JP8. This is the sound that made me realise I shouldn't have sold the synth. Main riff when beat comes in is JP4.
You Are Transparent by PolyDROID on SoundCloud - Hear the worldβs sounds
On this track, the sound that comes in at 0.44 is JP4. The sequence is System 100.
This System QE Mix by PolyDROID on SoundCloud - Hear the worldβs sounds
This track is mostly JP8, including the bass line. The riff that comes in at 1.10 is JP4. The Jp8 cant make a sound like that. All the detuned strings in the chorus are JP8 layered with DSi Mopho.
Enhance 224 QE Mix by PolyDROID on SoundCloud - Hear the worldβs sounds
Whats interesting is that whilst I kind of prefer the JP4, i've used the JP8 more actually.
Hope this helps.
Oh, and dont buy a JP6. Really not worth the money. A Juno 60 would serve you better. I had mine for a few months and shifted it. Never used it on one recording. It has no distinctive personality.
Brian