Q.4. Sphere : ayatana : sphere, base, region.
A term with a wide range of meanings, see old PTS dictionary page 105. The two main uses are in connection with :
1. The four formless absorptions (arupa-jjhana) (arupayatana).
2. The internal and external sense-bases or sense-spheres.
This passage may be open to as many as three interpretations :
1. A puthujjana monk may see it as a description of the highest concentration state - called "the cessation of perception and feeling". This is an unconscious state. This state may sometimes be thought to be nibbana - it is not.
2. As how things appear to an arahant. The four elements have ceased, the three realms have ceased. A description of nibbana element with residue remaining. ( this world = kama-dhatu, world beyond = rupa-dhatu). The sun and moon are a symbolic reference to vedic cosmology.
3. As how things appear to a sambhodi/tathagata. This interpretation takes "this world" and "world beyond" in a different sense. Here "this world" means all three realms. This is the nibbana element with no residue remaining.
Best wishes, Vincent.