Is it the change to fourth or the torque converter lockup? If you're not used to the box, it feels like you're in third, whereas you are in 4th but the lockup has not kicked in yet. Mine changes from 3rd to 4th at about 40 (unless you're accelarating hard), and then torque converter locks (to give direct drive) at just over 50mph. If you drive at a steady 50 and move the selector lever from drive to 3rd, you should notice the downshift.
The torque lockup is temperature dependent - based on the outside temperature, a timer decides when the transmission oil is properly warmed up, and only then will it activate. On a cold morning, it takes about 5-6 miles for this to kick in.
If it's not that and a new box, it doesn't sound like it's working correctly - it's almost as if you're in permanent sports mode, which would change the behaviour to what you are getting. Does that sports mode switch change the setting?
Oh, maybe check that there's enough oil in the box!
Cheers!