My pc not working and gives 4 beep only

AMI BIOS
4 short beeps = System timer failure

IBM BIOS
One Long and Three Short Beeps.= Video (EGA) Display Circuitry.

If your computer has an AMI BIOS then the System timer failure is a motherboard problem.
If your computer is an IBM BIOS then it is a video card problem. Try reseating the video card and if it dosen't fix your problem then replace the video card.

Most probably, this could be a videocard or memory problem.

Try opening your CPU and take out the memory and place it to a vacant slot. Make sure it is properly installed.

Do the same also with your videocard.

If it does not work, try isolating the problem isolating the problem. If you have extra memory or videocard, it is best to do trial and error by replacing one at a time to determine whether the memory or the video card is defective.