No beeps but lights A B are orange - Dell Dimension 4100 (-JM0TB01) PC Desktop

....and lights C, D are green, and your computer fails to boot up ? (you do not say) (on Dell PCs, which I don't have, the LEDs are usually described as yellow and green. There are four of them on rear of the PC ?).
First go to this webpage:
Product Support
if you are able run the Dell Diagnostics, or if not the ePSA.
Also read this manual - the only one available specifically for Dimension 4100 -
http://downloads.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-4100_Service%20Manual_en-us.pdf especially the section on safety. If you have to open the case remember to properly earth yourself.
To the answer to your question, if the 4600 model has the same codes as the 4100 see the service manual for the Dimension 4600 (or get the corresponding manual for the 4100 by contacting Dell support) look here
http://downloads.dell.com/Manuals/all-products/esuprt_desktop/esuprt_dimension_desktops/dimension-4600_service%20manual_en-us.pdf
in the Advanced troubleshooting section. It tells you the problem may be with your memory module(s) and it suggests what to try. I favour checking to see if you get correct errors with no memory inserted then with each module in each slot until all 4 combinations have been exhausted. This likely to tell you what is wrong. You may have to replace a memory module, or work with only one module or replace the motherboard.

23.June.2015