If you can get the 8gb ram its a huge difference in my opinion. Your system is only as strong as the lowest common denominator. In the I5 4gb, its the Ram. I just exchanged my system for the 8gb and its a major difference.
To quantify -
Greater performance in tools such as Sony Movie Maker 13, adobe, and MS office suite.
Slightly higher fps in games (difficult to quantify) but definitely smoother performance in all games so far.
I also added the Artic flex usb fan and mini usb expansion and it rocks! I can now stay at max turbo with GPU set at max while gaming.
Bf4 completely playable at low settings and > 37fps on full 64 player MP maps in even the most intense situations with smoke, particles, etc..
Fifa 2016 - playable at 60fps in low settings. Impressive!
Marvel Heroes 2016 - Low settings, actually playable now at over 30fps even in midtown and one shots.