No display but 4 lights

Just got a iMac emc 2134 a1225 24inch. Does the normal startup beep when you turn it but there is no display! Opened it up and I see 4 green less. All four lights are on… Initially 3 turn on, and then 2 seconds later the 4th comes on too. Fan is pretty loud but maybe that is normal?

What could the problem be?