The Photon Mono 4’s touchscreen shows a message when something blocks a print: a UV exposure fault, a bad file, a leveling problem, and so on. Anycubic has not published a single fixed numeric code list for this model the way some FDM printer makers do, so treat the exact on-screen wording, not a guessed code number, as the thing to search and act on.
Write down the full message before touching anything else. Check Anycubic’s official Photon Mono 4 wiki and FAQ pages first. They cover leveling and firmware-related issues directly and get updated as new firmware ships. A message tied to leveling should send you to the official leveling guide rather than to a generic recalibration attempt.
Start with reversible checks that match the message. Reseat the USB drive for a file-read error. Confirm the vat and FEP are seated and undamaged for an exposure or motion fault. Confirm the screen and light source are clean if the message describes an uneven or failed exposure. Retry once after the check. If the same message comes back, treat it as a hardware issue rather than a one-off glitch.
Update firmware only through Anycubic’s official firmware update guide, and never interrupt an update once it has started. An outdated firmware version can itself be the cause of a message that a current release has already fixed.
Stop and do not restart the print for anything involving a burning smell, visible smoke, a damaged power cable, or a screen that stays dark under exposure. Those point to a light-source or electrical fault, not something a factory reset solves. Keep the exact message, and the firmware version if asked, on hand when you contact Anycubic support.
Many of the messages covered here trace back to routine upkeep, so the maintenance schedule is a good place to start for prevention. If the message points specifically to failed layer separation or a damaged vat film, the FEP film replacement guide covers that repair directly.