#4 CU is correct.
250.66(B) Connections to Concrete-Encased Electrodes. Where
the grounding electrode conductor is connected to a single
or multiple concrete-encased electrode(s) as permitted in
250.52(A)(3), that portion of the conductor that is the sole
connection to the grounding electrode(s) shall not be required
to be larger than 4 AWG copper wire.