The problem with the "this scale for that chord" thinking is, how do you make it smooth, and not just sound like you're jumping from one scale to another? What's the setup?
I heard this little tip years ago, and worked with it for a while, and found it to be a reasonably solid approach:
"Transition from the third or seventh of the chord you're currently on, to the third or seventh of the chord you are moving to."
(On a side note...haha pun intended...I've been told that avoiding the tonic of the chord you're moving to is a way to avoid being predictable...).
This is a great way to be able to improvise successfully without having to do a lot of computation, and it sets up your moves from one scale/mode to another in a sonically valid way. You don't want to just randomly move from any note your finger happens to be on to the next scale, you may be hitting a note (in either the from, or to, scale) that is not ideal for the transition.
Let's take your chords:
Amin, G#Dim7, Gmin7, Csus4
You could take the chord tones and try to paste together some kind of common scale, but in my experience, that's not typically successful.
Instead, maybe think this way:
I'm noodle around in Amin. Here comes the transition. I have options:
- Slide from wherever I am in A minor to the C (third of Amin), tap the F (or just hit that interval), which puts me at the seventh of the G# (and definitely reinforces the harmony you're moving towards in the Gmin and Csus4).
- Bend the B up to the C (so the ninth to the third of Amin), then release the bend to the B (the third of the G#).
Both are reasonable sonic transitions to check out.
Now in the land of G#dim, since you know all the chord intervals in the dim chord are minor, you have fun with that for a while. Do the "minor third walk" up the strings, play around in whole/half, or what have you.
Now here comes the Gmin. Find either the B or the F, and you're looking for the third or the seventh of the next chord (Bb or F). You can ghost bend the B to the Bb, or just hang on the F (very nice, a sustain across chords).
Now you're set up in Gmin: you know the interval from the third to the fifth of a minor chord is a major third, you know the seventh to the tonic is a whole step, and all that. Use the intervals to set yourself up for your stock scale run, maybe a burn through G Dorian, and so on.
Here we go again, Gmin to Csus4. Third or seventh of current chord (Bb F) to third or seventh of target chord (E B). To add interest here, extend the 3/7 to 3/7 rule to include the tension (the F of the Csus4). Maybe grab the F of Gmin, and just sustain it to the transition (putting you at fourth of the Csus), then find a B and bend up to the root of the C.
Now, some might say, "you'll never figure out those notes on the fly."
Well...yes, I will. I know the notes on the neck well enough to do it. But, you don't necessarily have to think in terms of notes, and if you're really burning, the note computations may slow you down (not always a bad thing, but let's say you're in the mood for some shred).
But I ALWAYS know, in any given pattern I'm familiar with, where the third and sevenths are in relation the root, even if I don't know the notes. Anywhere I can find the root of that scale/mode on the neck, I automatically know where at least one third and seventh are; the seventh is either 1/2/3 frets down from the root (maj/min/dim), and the third is either a minor or major third above it. And with some practice, that can be computed with lightning speed. So finding a third or seventh of my current scale/mode, to the third of seventh of the target, doesn't have to be about notes. And once I've made the jump, I can burn away in the new pattern.
(If you have not developed the ability to look at a scale and know where the third, fifth, and seventh are in relation to the root, and eventually ideally to each other, you should. It is a VERY powerful ability).
That's how I might think of it.