Hi Alison,
It's more or less your call.  You alone can determine if you are faster and
better than you were four years ago.  I noticed after a 'seasoning' period
of about five years that I really was gaining some speed, intuition about
clients and an elevated sense that quality was indeed improving in my work.

Those factors gave me the confidence to raise my rates.  Interacting with
illustrators in my area clued me in to approximate going rates, and agood
pece of advi ce that I often repeat--if you find yourself too busy, raise
your rates! In other words, you'll know when to raise rates!

Best,
Chris