I think your problem is that in the "Configuration" section of the software you have the "File System" set for ISO9660. This standard has a 2GB limit. You can write 4 gb to DVD if you use UDF . Not sure which one, since there are several there to choose from........probably the latest version. Note: Not all computer DVD drives will support UDF.
I'm not entirely sure about all this because that's what used to happen to me a few years ago.
Google search for: 2GB limit on DVD discs