We strongly recommend you refer to your OPTIPLEX 760 manual for troubleshooting. Here is the URL: ftp://ftp.dell.com/Manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-760_Service%20Manual_en-us.pdf
Lights 3,4 indicate a RAM failure. To troubleshoot this, remove all modules, then insert them individually until the bad one shows a failure.
I have corrected your subject to reflect the model name.