Partiton Magic "should" do what you want, it does work with ext3 not just ext2. Ive used it before to resize & move linux partitions with out any problems, so you should be fine.

So I have tried Gparted both on the live CD and as an application running under Ubuntu. What I am trying to do is delete my NTFS partition (which I can easily do) and then resize my ext4 partition so ...

Before you can partition your disk you'll need to squeeze your Windows partition to free up some disk space for the new partition. Head to the Disk Management tool, and right-click your main partition ...