My Dell 4700 is showing the following diagnostic lights

A=Green
B=Amber
C=Green
D=Green

The light on the front of the pc comes on and I here the fans power on but I don't get anything on the monitor. I have tried both the video card and the on board video. I've taken all the cards out and replaced them one by one with no result. Please help.

Hello, and welcome to Geeks To Go. My name is hippiefreakcomputergeek (hfcg) and I will try to help you. First, have you tried to remove the memory and reseat ir? sometimes the memory will not make a good connection because of dust or shifting and you will not get video. (most video use shared RAM) when the memory is out please try to boot and see if you get a set of beep-beep-beep-ect.... If not than your system board (mother board) may be bad.
If you are getting lights and your fans spin then you are getting power (does not mean you are getting the proper amount of power though) but no beep at post I am afraid it may be your mother board or CPU. Your mother board would probally go before your CPU but lets try what I asked for first.

I did what you suggested. When I turned the pc on without ram it did the same thing. I didn't get any error beeps or messages.

There are no codes for the sequence of lights that you gave. The closest is a memory issue.
Look at the following. I have found your manual, and another code description.

MANUAL
MORE INFO.

Have you changed anything recently? Added any software, hardware?
You could try memtest, found HERE. You can create a floppy or a CD as an ISO.

If you have more that 1 stick of ram, try only 1.

OK. I am sorry to tell you that I believe that your mother board may be bad. If you feel that the computer is worth fixing you can take it to a repair shop for a quote, or try to change the mother board yourself. (If it was your CPU you should have gotten an error code beeping)

OK to be more specific.
When you have no memory installed, on post, the computer will give several beeps.
You have power because the fans spin and you have lights.
If the CPU is bad you should get high/low beeps.
No memory, no beeping= bad motherboard.