Oh, glad to hear about the termites not being a big problem. I think the rest is just up to you. If you dig down the top soil (at least the grass layer at minimum), you'll likely have your 4 inches of perimeter right there.

If you chose to not take out the top soil, you have the option of extending your gravel "pad" by a foot or so all around. This will naturally taper off to grass and likely wouldn't be an issue later. But if you wanted a cleaner look, I'm confident that your idea of using pressure treated lumber or blocks would work well.

Your gravel will likely not put much pressure against the lumber or blocks and push them out.