Hi, I'm Pete. Welcome to JustAnswer! I'm reviewing your question now, and will post back with your reply ASAP.
You will need to check and change the default paper settings on your computer.
If the printer is asking for letter paper it means your settings are using letter as default.
Change the settings to use A4 as standard, remove the current printer. Restart the computer, reload the printer and it should now work ok.
There may be a setting on one of the printer's menu that allows you to override A4/letter which sometimes will still allow you to use A4 without this problem.
Let me know how you get on. I can also provide remote assistance if required.