2012 - Obama’s youth vote came down to 60%.
The 4 million white voters who “stayed home” in 2012 are most likely young whites who voted for Obama in 2008.
But, as you say, long term, the political future for Conservatives looks very grim.
It isn’t purely a matter of race, the white percentages used to be a little higher, and Romney’s turnout was slightly higher than McCains’s, not 4 million lower.
I’m not aware of anything showing that the white youth stayed home in 2012.