My Lowes landscape timbers are green pressure treated usually #3 grade southern yellow pine. They are an actual 2.75 x 3.88 inches in cross section. Green means that they are not kiln dried and are “wet.” They require a long time to dry enough for paint to reliably adhere. As they dry they are prone to splitting, checking, cracking, and warping.
Lowes 4x4’s are pressure treated grade #2 southern yellow pine, meaning they have fewer defects and knots (weak points). They are an actual 3.5 x 3.5 inches in cross section, so stronger and less likely to break. After pressure treatment they are kiln dried, so are paint-ready sooner and less likely to deform.
About 85 percent of all pressure treated lumber in the US is southern yellow pine. But you might live in the western part of the country where your pressure treated lumber is Douglas fir.
Cost for a Lowes 4x4x8 is $11.68 and an 8 foot landscape timber is $4.88, both at today’s prices. Landscape timbers seem to only come from my Lowe’s as 8 foot lengths. I’d prefer 10 feet for a home arena jump rail.
That all is why I choose to pay more for my jump rail lumber and make octagons from 4x4x10 kiln dried pressure treated sticks. Better grade wood, stronger, less likely to deform, and paintable much sooner.