Just my experience ...drove a 2010 XL 4.6 a few months ago....with the 4 speed tranny in it....
I'm guessing as it was a previous city work truck & as I understand it , it was an option to keep fleet trucks @ a much lower cost...

 

You are correct - I did not realize this. The 4 speed was used in the lower trims in 2009-2010 and then in 2011 they all switched to the 6 speed.

OP was looking at the Lariat trim - so definitely would have had the 6 speed for 2010.