If you want to install a 4' high fence you most likely will need 6' tall posts. You should plan to set your posts about 18-24 inches into the ground in concrete and let them sit for 24 hours so they can harden overnight. 6' tall fences can use 8' tall posts, anything over 6' and you should consider setting them deeper than 2' in the ground for additional stability.

Tubing is the thinnest post that you can find and is used primarily for the top rail on your chain link fence. Your posts are commonly structural and much thicker than the tubing used in the top rail. For commercial applications, consider getting schedule 40 for your posts, the thickest available this would be great for a gate that semis or big trucks use where there is a high risk of a vehicle crashing into it.

Your black posts will be powder coated so be careful when you're doing your installation to not chip off that layer of paint or you will expose the grey steel underneath. You can always touch up your posts if you need to, but take extra precaution and you won't have to waste your time.