Hi Oguz,
I understand your concern. The description for the DAC7800 in the datasheet says “The DAC7800 features a serial interface capable of clocking in data at a rate of at least 10MHz.” This is a typo, there is no minimum clock frequency requirement of 10MHz for the DAC7800.
If you look at page 4 of the datasheet under the timing characteristics, the minimum CLK pulse width is 40ns which implies a minimum clock period of 80ns, or a maximum CLK frequency of 12.5MHz, so a 4MHz clock is acceptable.
Regards,
Neeraj