Calibration on a resin printer covers two separate things: how flat the build plate sits relative to the screen, and how long each layer is exposed to light. Both need to be right before a print has a real chance of succeeding.
Platform leveling brings the build plate parallel to the LCD screen so the first layers press evenly across the whole surface instead of touching down on one corner first. Run the on-screen leveling routine before your first print and any time you reinstall the plate after removing it. Anycubic’s leveling guide for the Photon Mono 4 walks through the exact steps for this printer. Follow it directly rather than relying on a general resin-printer routine, since the procedure and screen prompts vary between models.
Exposure time is the second half of calibration. Each normal layer needs enough light through the LCD to cure the resin to the right thickness, while the bottom layers use a longer exposure to anchor the part firmly to the plate. Photon Workshop ships with presets tied to resin type and brand. Start from the preset matching your resin instead of guessing a value, and check the resin bottle or manufacturer page for a recommended exposure range if one is listed.
Recalibrate platform leveling whenever you move the printer, reseat the build plate, or notice prints releasing from the plate mid-job. Revisit exposure settings when you switch to a different resin type or brand, or when finished prints come out soft and underexposed, or overly thick with lost fine detail. Both point to an exposure time that no longer matches the resin in the vat.
If you replace the FEP film or the vat itself, run the leveling check again before your next print. The plate, vat and screen stack directly on top of each other, and small differences between them affect the very first layers most.
For the slicer side of exposure settings, the Photon Workshop profile setup walks through choosing and adjusting a resin exposure profile before you slice a model. If you skipped it earlier, the initial setup guide covers the first leveling pass this routine builds on.