| A sus chord can function a few different ways, but most simply -- yes, it has a 4th inplace of the 3rd.
Here're a few tips.
In a traditional harmonic place, where the sus chord resolves to a major chord a 4th up (G7sus to C), the G7sus can be approached like a D-7, a G7, or even a F triad over G (F/G). You can boil this down simply to the Ionian (root position major scale) of the parent key -- in this case, C. So, D dorian and G mixo-lydian are pretty useful.
If the situation is similar to the above, but the resolution is minor, then you can apply the same sort of approach, but take care that you are dealing in minor. Listen to the chordal instrument and melody to get a proper sense of how the chord is being used. So, for a G7sus (b9), you can approach the chord like a D-7(b5) or a G7 alt. Your base scale would be C minor (C aolian, based on Eb Ionian). You can approach this with D locrian, G phrygian (both just a Eb major scale, really), or even go further and get into some melodic minor and diminished things (which can also be used successfull above).
If the sus chord is static, frequently used in fusion and 60's modal kind of stuff, you can treat the sus chord as you would a regular ol' dominant 7th chord -- pentatonics and the like. | 
