Water losses are classified 1 through 4 under the IICRC S500 standard based on how much water the affected materials absorbed and how hard they are to dry. Class 4 is the deep end: significant water bound inside low-permeance, high-density materials — hardwood flooring, plaster, brick, and concrete assemblies.
Class 4 exists because dense materials don't give water back easily. A wet carpet releases moisture to moving air within days; a saturated slab or hardwood floor holds moisture deep inside, where ordinary airflow barely touches it. Drying them takes lower vapor pressure and longer exposure — desiccant dehumidification, specialty mat systems that pull moisture up through hardwood, and sometimes intra-wall or under-cabinet injection drying.
For homeowners, the class explains why two losses that look similar get very different equipment and timelines. It's also why 'it feels dry' is not closure: Class 4 assemblies can read dry at the surface while holding damaging moisture inside, which is why professionals verify against dry-standard readings with moisture meters before pulling equipment.
If a floor is worth saving, class matters early: hardwood caught quickly and put under mat drying can often be rescued; the same floor left a week usually cups beyond saving. That save-it-or-replace-it fork is a scope and cost inflection point on any claim.