depends what you do?
I've got 12GB of triple channel, if I'm gaming then RAM usage will stick around 4-5; if I'm editing photos or movies then it can quickly jump upto 10-11.
For gaming 4 is fine, but not that future proof, as a work station you'd really want 8, you'd probably find that adding an SSD would give you more of a noticeable increase than adding more RAM