@Minicube
There is a slight difference but it certainly is not as night and day.
@OP
For many people adding a 4GB to their system for a 1+4 arrangement is an inexpensive upgrade. I did 8GB because we have up to four users, sometimes all logged in, and two of those are gamers. I bought my RAM early on but now prices are really nice.
I haven't bought through Crucial before. I have used Newegg and OWC. I have heard of some people speccing out the wrong ram at Newegg; not Neweggs fault. I have bought from OWC a few times and their RAM always works. As with all ram, all the time, keep an eye on pricing because it continually changes and usually goes down.