Bluescreen after changing RAM

window 7 home premium 64 bit
add up new 4gb RAM

bluescreen start to occur after changing RAM..
why it happen?

looks likely memory was sold damaged, or manufacturer error.

if you want to check the memory yourself. There are good instructions for that here:
http://www.sevenforums.com/tutorials/105647-ram-test-memtest86.html

You may be in a return situation with your new ram here.
Best to verify with the procedure linked above though.