H4 is appropriate for less critical ‘non-structural’ applications and/or where the decay hazard zone indicates a lower in-ground hazard.
Timbers allocated H4 (Hazard level 4) can only be used in the specified application if that timber can be commercially treated to a minimum H4 level.
N.B. Currently, the heartwood of most hardwood timbers and some softwood timbers cannot be treated to this level of protection. The timber treater is to ensure that their process has achieved the required level of penetration to meet the stated H level specification.