Memory modules are detected, but a memory failure has occurred.
According to Dell lights 3 and 4 ^
http://www.redplanettrading.com/ebay/desktops/case/optiplex780/Optiplex780_SM_DT/diags.htm#diaglight
If two or more memory modules are installed, remove the modules, then reinstall one module and restart the computer. If the computer starts normally, continue to install additional memory modules (one at a time) until you have identified a faulty module or reinstalled all modules without error.
If available, install working memory of the same type into your computer.
If the problem persists, contact Dell.