Q: 4 digit code.

No one but you knows the code. If you cannot remember it, then you need to restore the computer from scratch:

Install El Capitan from Scratch

 

     Backup if possible before continuing.

 

1. Restart the computer. Immediately after the chime hold down the Command and R keys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Reinstall OS X and click on the Continue button.

What 4-digit code are you referring to? I've never had to enter one when powering on my Macs. Knowing what your are referring to might help; however, as Kappy points out, there is no way to "recover" any security "code" on OS X.

You can reset user account passwords, but just about anything else security-wise is non-recoverable.

A four digit code will appear if the Mac has been locked remotely: iCloud: Use Lost Mode.

"Lost mode" is intended to keep that Mac's information secure in the event it becomes lost or stolen. It is very secure. There is no "back door" and no way to circumvent it. An Apple Store will be unable to bypass it, even if you are that Mac's legitimate owner and can prove it.

If you do not know that code, your only option is to erase the Mac.