Magical Knack can raise the magus's magus caster level, but the trait doesn't then raise the magus's magus level
Magus arcana have prerequisites that must be met, and many times this prerequisite is listed as, for example, Magus 9, like for the magus arcana accurate strike. This is the minimum magus level a creature must have to pick the arcana. The trait Magical Knack only increases a creature's caster level for the picked class, not a creature's effective level for the whole class.
This means a level 4 magus—even one with the Magical Knack (magus)—can only meet the magus arcana prerequisites of magus arcana that don't have a prerequisite of magus level 5 or higher, like arcane accuracy or arcane cloak.