Further insight into Earth/human ascension

[seek]

I was reading through the 2011 transcripts and came across an interesting concept eluded to by Q'uo. What was stated was that indeed the work to bring Earth into 4th+ was complete but work remained for those that wished to strengthen the harvests numbers and combat the negative whom wish to further their agenda (balance and whatnot).

My question[s] I guess is/are does anyone have any further insight to this? I mean how can 3rd density beings exist on a 4th density planet? Does this mean that this current incarnation is the last one on this planet if we are not harvestable 4th+? What about children born after Dec 21 2012 (or whatever actual day the planetary harvest occurred)? Are they already 4th+? I guess I just assumed that some abrupt end to 3rd density would occur instead of a gradual die off.

P.S. What day does Q'uo state was the actual planetary harvest?