Originally Posted by: Swe_ShredderIf I want to solo over a rock/metal riff that goes Em , G , Am , Em

Lets say i decide to solo in pentatonic. I can just use the E minor pentatonic scale all the way? Or do I have to change to G major penta and Am penta when those chords are played?

With those chords you could use an "E minor Pentatonic" or an "E minor scale"
Or you could take the [U]chord by chord approach[/U], just determine the quality of the chord over which you are playing(major, minor, diminished, etc),
and play whatever scale or mode is compatible with that chord.