I would have to say Kif is giving sound advice... What is this proposed life span? If you are paying this much money and it is suppose to be a laptop replacement sometime in the future *cough* Windows 9 *cough* its very likely you will run into needing more ram. Having more ram is always sound advise for any laptop or computer unless you are using this just as a tablet and if you are doing that I would advise saving your money and buying something like the Galaxy Note Pro 12.2
Why do you believe windows 9 is going to require more ram? Do you have any reason to actually think that? Bare in mind that from vista to 8 each release has required less and less ram..
This proposed lifespan was as in my requirement for this device prior to replacing. It was to point out that at the time of his posting, your own needs were not divulged, and as such simply saying you'll be glad to have the ram is a worthless statement without any context.
And as I've said, the machine is no laptop replacement, ultrabook maybe. It is not and will not be a workhorse in this iteration. The only software I would wish to run that might actually require 8 over 4, the machine is not capable of running to start with, and windows 9 would not be able to change that.