My understanding is that H4 is an Australian grade which describes a treated post for in-ground usage and sized 100mm x 100mm nominal (approximately a 4” x 4” in the US).
Is this a Kiln Dry After Treatment (KDAT) process? If so, were the timbers treated with a full cell process, leaving a lot of liquid in the wood cells, and resulting in a longer drying time than the green lumber (before preservative treatment)?
It may simply be that drying 4” thick pine at 90C dry bulb/ 60C wet bulb (this is 194F / 140F) for 70 hours is simply not long enough. KDAT of southern yellow pine is often not done if it cannot be done at temperature above 212F because of the lengthy time required.
Importantly, with this very wet material, you need to make sure that you are maximizing air flow efficiency by baffling the kiln load well to insure that the air flow is moving through the stack (and not around the ends of the stacks). This will both speed drying and minimize moisture variability.