Eagle Hierarchical Modules: Multiple Instances Automatically Sharing Multi-gate Component?
If you are going to use an existing part, then you must pick a part that has multiple gates, such as the quad op-amp LM324.  Click the Add icon from the toolbar (below the wrench) and select your desired part and package.  Then each time you click on the schematic, one gate will be added:



To get the power pins, right-click one of the gates and issue the Invoke command:



and select the PWR+- line.  This will place the + and - power pins on the page:



You can also use the Invoke command to add additional gates from the device, if you didn't do so all at once when initially adding the part.

On the other hand, if you are going to create a new part in the a library that has multiple gates, then you simply add the same gate as many times as needed  at the device level, for example choosing a OPAMP symbol four times like this, and then choosing the PWR+- symbol and place it over the first OPAMP symbol:



Note that the PWR+- symbol is added with the Add Level "request".  The other gates are automatically added with the Add Level "next".

Of course you can add custom symbols as needed before creating the device.