Transmission In 4th Gear??

[salstrip]

hi,
have had the latest g30 flash ,the vehicle 6.7 auto ,now sometimes whilst driving will switch to 4th on the display on the speedo and be locked in 4th gear ,you cannot get it to move from here ,you can flick the toggle on the shifter and it wont move , you can change it back to park or reverse or neutral and then shift it to drive and it goes straight to 4th .it seems to be in 4th gear as it will not go up a hill from stopped.if you switch off the vehicle and start it again all is good again and you can shift up and down the gears .
anyone help please
regards conn