Looking at the situation currently, PS4 would almost have to sell 1 Mil a week in December to reach 20M by 01.01.15
How do you expect that to happen? Is there historical launch data that suggests this is possible? I play for the blue team and I badly want to see Playstations continued success but 20M looks ambitious today.
History shows that generally Jan-August/September = September/October-December... Going by the PS4 announcement on August and sales on August+September and estimating end year on that basis gave 21M PS4 (which seems excessive because of possible lack of stock and possibly demand) but 20M is feasible altough 19M is the "guaranteed" number...
Several times the top selling console gets 1M-1.5M(altough this extreme just Wii got once) on Black friday and can get another 3-4M in December. So keep your hopes up.