Someone was spreading misinformation.
The dag file is just barely over 2gb as of today, just checked my miner.
So 4gb cards will be good for another estimated 1 year 9 months.
IMO ethereum will go pos by then and mining will suck OR you will be looking into upgrading to newer faster cards by then.
There is no benefit in spending the extra up front aside from mining some shit coins that require 8gb of ram.
8gb choice has nothing to do with the dag file , 8gb cards after a year as you are calculating will have more resell value than 4gb so less in risk factor, what happen if eth turned pos and one of those shitcoins firedup to moon then you can't mine it with 4gb but for 8gb sure you can and you can see this in the 2gb cards now.