Sql Server 2016 maintenance plan with weekly full backup and 4h differential backups
Sounds like you are using maintenance plans for your Full backup.  If so, make a new plan with a Backup task in it and change the backup type in the GUI to Differential, then set the schedule.   As RDFozz says, consider also using t-log backups every 15-60 minutes throughout the day.   If not, and your databases are in FULL recovery model, you will likely get crazy file growth in your .ldf files :)



Note that the task in this image says full, because that is the default.  When I click OK, it will change.