I link you a thread with some helpsites that may help you:
If you can't disable secure boot in your BIOS (that would be the easiest way) you can try the steps shown there. With mok keys I'm not familar.
If you have a Dualboot with Windows also disable fast startup in the energy settings of Windows, and also turn off fast boot in your BIOS.
It may also help to switch to Xorg. Logout and at login click on your username then a cog wheal appears at the bottom right corner. There select "Zorin Desktop on Xorg".
4 GiB RAM are low for running Zorin core.
If you continue to experience problems with your system freezing or not running smoothly, try Zorin lite. Zorin 18 lite has not been released yet, but you could try Zorin 17 lite.