Watch the Modal Study #3 online guitar lesson by Jon Finn from Modal Rock Soloing
C mixolydian is an F major scale starting from its 5th degree (C). C mixolydian is a C major scale with a b7 (Bb). C mixolydian is a C major pentatonic scale with a 4 (F) and b7th (Bb) added. Since mixolydian is the only major sounding mode with a b7, featuring that note will give you a definite mixolydian sound. You can play a 4th within the phrase and it will sound fine. Just not exclusively mixolydian if that makes sense. The basic idea is to try to understand the same information from more than one point of view.