Before replacing toners in Brother printers, it is recommended that you try resetting the Toner counters on the printer to attempt to extend the life of the existing toner.
Brother MFC-9840CDW:
1) Open the front cover but do not remove the toner catridges
2) Press the Clear/Back button until the display shows the reset options (2 for each toner type)
3) Select one of the options and press select again to reset.
4) Repeat the process for all 8 options
5) Close the front cover and check if the toner still needs to be replaced.