To get the Unreal Engine 4 offline installer:
Once you have the offline folder (the raw chunks), compress it using with Ultra compression. Unreal Engine 4 Download Offline Installer
If Epic Launcher insists on logging in when you have no internet: To get the Unreal Engine 4 offline installer:
Epic Games Launcher stores downloaded engine files before installation. You can hijack this cache. Unreal Engine 4 Download Offline Installer