Download the 4GB Patch from here: https://ntcore.com/4gb-patch/, or use the one included when you download NAM.
Unzip the download and run the “4gb_patch.exe” as an administrator (Right-click > Run as administrator)

When the window appears, navigate to your SimCity 4 installation directory
Select the “SimCity 4.exe”
Click “Open” to start patching the .exe
Click “OK” when the patch is successful.
Test and run your game to see if it’s working properly.