Ok I have to write from experience;
I have the MBA 2014 11" with 256 and 4 gig memory. My desktop is a 2012 Mini i7 Quad core (the fast one), 1 t HD, 256 SSD drives and 16 gig memory.
I use the 11" air on the road almost every day. This past week I made a 3 min video using iMovie for the web, running Photoshop elements, Firefox, a FTP down loader and text editor all at once on the 11." With 4 gigs it ran really good and only bogged down when I tried to access a network drive via wireless connection.
For those nay-Sayers who say "8 gigs," I would bet they do very little "power" computing on their machines. The biggest bottle necks on all computers these days are the storage access, not the normal memory. This means if your program needs to do a lot of drive read and writing, like a video program, the faster the better!
BTW I rendered the video both on the Mini and the 11" MBA, there's a significant difference in CPU speed between the two, and noticed very little difference in processing speed of the video. Where I did notice a huge difference is when I imported the movie to Final Cut Pro X and then exported it to a file.
FCPX renders videos on the fly rather than at the end, as is the case with iMovie, therefore exporting to a file was a lot faster in FCPX than in iMovie on the same machine.
I will be in line to get the 12" MBA when it comes out.
Enjoy!