Jagat Mithya, Bramha Satya

 Upanishad (Mandukya etc.) primarily divided reality into 4 divisions without confusing with lokaa. 

1 Jagriti

Waking state where experience we perceive by sensory organs take place. Also, if we divide the total life per day into 3 equal 8 hours then 2 part every day falls in this part. Anyone's 2/3 life is spent in this state. Also, physical realities or interaction with matter take place in this state due to physical activity. Here, jagriti is experience related reality rather than physical reality of the universe. 

2 Swapna

In absence of access to Suspti or Turiya, repetition of what has been perceived in Jagriti state. Also, ones desire etc roam around in this state. One need not be in sleep, though during sleep this state is prominent and hence in our vague approximation 1/3 of life is spent in this state. Though all mental activities or replication of past experiences is dream. Which is not present experience nor experience at all but the remembrance of something happened or wished to happen.

3 Susupti

This one is absence of 1 and 2. It can be termed as Sunya. One can confuse this with highest as nothing exist here or due to inability to recollect any activity as in 1 or 2, it can be thought of as ultimate. Since, this state is also perceived by something else it can not be the highest. Neither senses of 1, nor mind of 2 can access it. Only pure observer takes account of it.

4 Turiya

Highest or the observer who can account 1,2 and 3 simultaneously and yet unattached to any one of those states. This is the Bramha. 

why 4 alone is reality?

So explained by Adi Guru Shanakaracharya (as much as I understand), since all 1,2 and 3 does not have beginning or an end, it is fictional. All these 3 Jagat are therefore Mithya or unreal. Unreal not by experience but by their relative position from 4. Since, 4 originate 1,2,3 and some even extend all matter too originate by 4 (still I am not sure about it) but it is true all these experiences are originate from something other than 1,2 and 3. Very much  observable at time of birth and death, something came experienced and be gone. But who is this real, who experience essence of lives in these 3 states. May be by same logic it must originated matter for its experience. Thereby becoming the Sadhan, Sadhana, Sadhak  nothing but one Sadhya.