Hi,
Well 4 seconds in 2.2mb page size on the homepage that’s a good speed, also notice the Woocommerce pages a little bit slower but since it has to gather more information it’s reasonable that it takes a bit more time.
If you already applied all those procedures of optimization that you described above, then the only thing that can be done it’s hiring a faster hosting(this one is already fast in my opinion).
We work with the guys from WP-Engine that are great, faster then them there are the Kinsta guys(https://kinsta.com/)
But the prices also start rising with the speed.
Hope my answer helps.
-Rui