Of course, a beautiful solo is made up of a lot of things.
But the one issue you're talking about falls in the category of 'dexterity'.
Basically you want to make sure the you have total finger independence. You want to make sure that you can count each of your fingers equally.
You need a workout that will work your fingers out equally. Songs and scales won't do that. A good place to start is a permutation chart based on the number '4' since you have 4 finger. Basically there are 24 combinations(permutations) that you can move your 4 fingers in. You want to be equally comfortable with all 24. And you need make sure that you're doing these permutations across 1 string, 2 strings, 3 strings and 4 strings if you play 4 string bass.
The chart is based on math, not bass. It's a chart based on 'possibilities'.
I'm sure if you google permutation chart you'll find the 24 variations. And I'm sure that someone will read this and tell you exactly where to find one.
peace,
anthony