The lifespan of a 4x4 post in the ground is variable and is highly dependent on the type of wood, its treatment, the soil conditions, and the area's climate. \r In general, untreated 4x4 posts made from softwood such as pine or spruce will last approximately 3 to 7 years in the ground before they begin to rot and decay.\r However, pressure-treated 4x4 posts can last significantly longer. Pressure-treated timber is coated with preservatives to protect against rot, decay, and insect damage, extending the lifespan of the wood. \r The specific lifespan of pressure-treated 4x4 wooden posts will depend on the treatment process and local conditions, but they can last between 20-25 years or more. However, even pressure-treated wood will eventually decay and should be inspected periodically for signs of rot or damage. \r You can extend the lifespan of your 4x4 wooden posts by installing them correctly. Set the posts to the appropriate depth and use suitable backfill materials for additional support.