While everybody’s body is different 20:4 fasting is not likely to induce autophagy to a significant extent. Generally, it is believed it takes at least 24 hours and some people don’t get into it until past the 36 hour mark. Autophagy in humans is extremely difficult to study so most of what we know is from inferences based upon studies in mice.
As for what people feel when in that state again it varies widely by person. Some people don’t consciously experience anything at all.
One should never do IF for autophagy alone, it should be viewed as side benefit. If it happens, it happens; if it doesn’t no big deal.