The amount between Shipped and Sell-through is never completely consistent so unless there's a massive difference, or almost no difference, we can't make any assumptions one way or the other.

We can look at dates when Sony gave Shipped/Consumer figures close to each other and compare what the difference has been officially during the PS4's life.

Date (First date is sell-through, second is shipped) Sell-Through Shipped Difference Days Between Announcements
December 28 2013 – December 31 2013 4.2m 4.5m 0.3m 3
April 6 2014 – March 31 2014 7m 7.5m 0.5m 6
January 4 2015 – December 31 2014 18.5m 19.9m 1.4m 4
January 3 2016 – December 31 2015 35.9m 37.7m 1.8m 3
January 1 2017 – December 31 2016 53.4m 57.1m 3.7m 1
December 31 2017 – December 31 2017 73.6m 76.5m 2.9m 0

Unfortunately most of the figures we have recently are for holiday figures, the 3.7m figure is that high due to the Pro being launched and needing to stock an additional model.

2.6m does seem like it could be a bit high for mid-september, as you'd expect a larger discrepancy between shipped and sell-through afterwards on December 31st. So it's possible that it is undertracked by a few hundred thousands, but it's also possible that the figures are fine.

Basically we have no way of knowing until Sony announce sell-through numbers. The only real inference we can make is that PS4 certainly isn't overtracked by a significant amount.

LTD: PS4 - 125m, Switch - 110m, XBO - 51m

2020: PS4 - 9m, Switch - 22.5m, XBO - 2.5m, PS5 - 4.5m, XBX - 2.8m