driving and a 4 is in place of d for drive and in left corner is the letter B - i was towing a trailer when this showed up

"B" is for odometer trip "B" I suspect the 4 indicates the gear selected with the shift lever in manual shift mode? https://owners.nissanusa.com/content/techpub/ManualsAndGuides/Frontier/2016/2016-Frontier-owner-manual.pdf Check the owners manual to familiarize yourself with the operation.