I've collected examples of PS3 & 360 sales in each year, showing (Sep-Dec) / (Jan-Aug) sales. We can then estimate 2004 Sep-Dec sales range by applying similar ratios.
[PS4]
2007: 4586k / 3335k = 1.4
2008: 4750k / 5708k = 0.8 <- Price cut effect & MGS4 boosted fist half of year
2009: 8894k / 4328k = 2.1 <- PS3 Slim in September
2010: 7043k / 6338k = 1.1
2011: 7734k / 6579k = 1.2
[360]
2006: 3687k / 3125k = 1.2
2007: 4823k / 3087k = 1.6 <- Price cut in August, Halo 3 in Sep.
2008: 6847k / 3855k = 1.8 <- Price cut in September
2009: 5810k / 4555k = 1.3
2010: 7911k / 5624k = 1.4
PS4 Jan-Aug 6.0M (my estimate)
XB1 Jan-Aug 2.3M (my estimate)
PS4 schedule until Dec is good but no price cuts, so I'm going to give it a conservative 1.2 ratio, meaning 7.2M units sold in Sep-Dec period, so my estimate is PS4 could reach 17.4 Million units by end of 2014.
XB1 has had an early price cut, and players are excited for upcoming games, however it already had it's biggest game of the year Titanfall. I'm going to give it a more generous ratio of 1.5, meaning 3.5M units sold in Sep-Dec period, so my estimate is XB1 could reach 8.8 Million units by end of 2014.
If I had to give ranges I'd say PS4 16.8M~19.8M and XB1 8.0M~9.0M.
What say you?
