No serial output (rev C4)

Hello, i recently received spare beagleboard C4 from my friend. But
device has some problems. There is no serial output on boot, but all
leds are ok. When i insert sdCard, Os gets start and i have clear
video output from hdmi.

What could be the problem with serial output?

Check out the serial connection:
You need a cable null-modem and the following connector
http://www.pccables.com/07120.htm

Check out U-boot arguments:
Take a look to the following link

Set-up your serial-terminal properly(minicom,putty...)
http://elinux.org/BeagleBoardBeginners#Setting_up_serial_connection_between_your_desktop_machine_and_the_BeagleBoard

Check out U-boot arguments:
Take a look to the following link http://code.google.com/p/beagleboard/wiki/BootingBeagleBoard