As Huckle said, booting from a live USB or live CD would probably make it possible. You can get what you need from the download site at ubuntu.com, and when you're ready you can boot into Ubuntu just as you're used to, and use GParted to move the partition to the left.
However, I would recommend another approach:
It looks as if the only partitioning you've done is into one Mac partition and one Ubuntu partition. Consider creating a third partition for your data. (That partition could even be shared between both operative systems, if you want.)
Using for example the Disk Analyzer utility, you can identify the largest node of your Ubuntu file tree, and copy that over to the new partition (let's call it
/dev/sda6, for the sake of this argument). When you have copied all the files you want, and verified that they were copied OK to the new partition, you can delete them from
/dev/sda4. Note! If you chose to move system files, such as
/home, you must reboot in single user mode before you delete the files!
Then, edit your
/etc/fstab to mount the new partition in the same place in the file system. For example, if
/home/cerin/ was the largest folder, I would suggest moving
cerin to the new partition and mount the partition at
/home with the following fstab line:
/dev/sda6 /home ext4 defaults 0 0
If you're unfamiliar with
/etc/fstab, this guide seems to give a good introduction. It hasn't been updated for a couple of years, but I suspect that the basics of fstab haven't either =)
While you're at it...
If it turns out your home folder isn't the largest folder in your file system, I would still recommend moving it aside on a separate partition. The main benefit of this is that if something fails on your Ubuntu system, you can reinstall Ubuntu from scratch and still keep most of your user settings. I'm running Ubuntu/Win7 dual booted on a 120GB SSD, and on that I've allocated 500MB for my home partition - it turns out this is on the edge of too little, and I've had to use some other quirks to work around this. I would (given that I know nothing of what files you have that's using up your space...) recommend a 40GB data partition and a 5GB home partition.