The first question is how big and how many video elements are you working with?
If you are doing something small you might be able to squeeze by with what you have now (small projects). You'll want to clear off as much as you can from your SSD (apps & data) as the app will need the every bit of space to work off of. That way the App has use of virtual RAM and paging space using your internal SSD. You'll still need an external USB3 drive to hold your data & project files. This won't be workable for anything very large.
If you had a 8GB model you'll have much more possibilities! While we use Mac Pro's with external RAID'ed Thunderbolt drives for our video editing. You can also do it that way with your system! Again, you'll want to strip off all of your data from the internal SSD (the leaner you are the better here!) as your SSD space is limited as well here (unless you also upgrade the SSD). That way the App has lots of virtual RAM and paging space using your internal SSD. Again here you'll want all of your data (what you currently have) as well as your projects sitting on the external RAID'ed drive.
Here are some possible options for your RAID drive system: