Imo a 4 man team is harder to make than a 6 ones. The group have to be very well balanced.
Tank + cleric are necessary, then you could go for dps only, or the other option : add a CC.
You have many choice possible. If i had to choose, i ll add a CC.
So that would make
-Tank + Cleric + bard + melee DPS
-Tank + Cleric + Enc + caster DPS