In Cyclone IV GX devices, the frequency of DCLK used in AS configuration can be selected.

It can be set by the following methods.
Select Assignments => Device => Device and Pin Options from the Quartus II main menu
Select Configuration from the Category column on the right side of the screen
From the Active serial clock source column
・ Select from CLKUSR, 20MHz Internal Oscillator, 40MHz Internal Oscillator

*supplement*
CLKUSR is selected when using the clock supplied to the CLKUSR pin as DCLK.