Home Depot sells steel spikes with the top end built to accept 4X4s.
They 2 to 3 feet long, are driven into ground with sledge hammer
(beating on wasted parts of 4X4), and are shaped to accept 4X4 wood
posts bolted onto the top. You would end up with nothing but steel in
the ground and whatever 4X4 wood posts you choose to use to build the
fence onto bolted onto the top of the steel spike. No levelling
problems, no termite problems, no digging holes and pouring concrete to
hold the 4X4s.