They are incorrect
In Rangers or B4000 the 4.0l OHV was VIN X(1990-2000), it would bolt in but computer couldn't run it well, injectors are not the issue
4.0l SOHC was VIN E for all Ranger's and B4000's, 2001 to 2011
Explorers used K and E for the 4.0l SOHC, K was for flex fuel model and yes you would need to change injectors if you put in a K engine, but it would work fine after that
Mustangs used VIN N for the 4.0l SOHC and it would also work for your swap, BUT...........you have to swap over upper and lower intake from the B4000 4.0l SOHC
Mustang used drive by wire not a throttle cable, and air intake was on opposite side