My Brother Hl 4040cn printer prints with a black/grey film over the sheets

SOURCE: Brother HL-4040CN won't read or show new refilled cartridge in the LCD

I think I know what the problem is...
When you refill these toner cartridges you need to reset the "toner empty" cog wheel.
For instructions on how to do this follw this link:
http://www.proprint.co.uk/brother_hl4040.htm
It has pictures and explains how to reset the cartridge
Good luck,
Billy.

SOURCE: Brother HL-4040CN Fuser Cover

Click this link below;

http://www.uninetimaging.com/newsletter/Brother_HL_4040_TN_115_Reman_eng.pdf

SOURCE: Brother HL-4040CN successfully installed but shows offline

I assume it's turned on and connected to the router.
Sometimes with DHCP, the IP address on the printer will change. I end up going into the router table to see what IP addresses are assigned where. Invariably I'll find my network printer has changed. Find the printer and write it down.

In Windows, Go into Printers> choose the Brother and right click to check Properties. Look for Ports and change the IP address as needed. It will come back online.

SOURCE: Difference between Brother HL-4040CN and HL-4050CDN printer

HL-4040CDN supports BR--Script (like Postscript) and
HL-4040CN does not.
So, there is a "postscript" diver for the 4040CDN, but not for the 4040CN.

SOURCE: how do you reset fuser life on a brother HL 4040cn

Press the Go and 'Up' buttons at the same time in the ready status.

The "Reset Parts Life" will appear on the LCD. Select the applicable periodical replacement part by pressing the 'Up' or 'Down' button and press the 'OK' button.