No. The 603 and 604 ranges look similar and the model numbers are only one digit apart, but they are not interchangeable. The XP-2100, XP-3100 and XP-4100 take 603XL; the XP-2200, XP-3200 and XP-4200 take 604XL. This catches people out regularly, so it's worth double-checking before ordering.

They fit exactly the same printers — the XL simply holds more ink. If you print often, 604XL works out cheaper per page and means fewer cartridge changes. For light use the standard 604 may be enough. You can run one of each in the same printer without any problem.

No. The 604XL range uses separate cartridges for each colour, so you only replace the one that has actually run out. A multipack normally works out cheaper than buying four singles, so it's worth a full set if you print regularly in colour.

Epson printers show a message whenever a cartridge that isn't Epson-branded is fitted. It's a notification rather than a fault. Confirm the prompt on the printer screen or on your computer and printing continues as normal. It may reappear after the printer has been switched off and on again.