I don't really care whether he is mentioned along side the Big 4. That is more a reflection of media coverage, hype and what most people watch, not necessarily an accurate measure of quality. He has performed better at a better average than Kohli and Root in the last 2 years. For me, a guy who averages 50 opening the batting mostly in foreign conditions in the last 6 months, including England and Australia, two of the most difficult conditions for an Asian batsman, and has scored a triple and a double in that time is a match winner.
Yes he has weaknesses rotating the strike and can get bogged down but every one has different weaknesses. Kohli struggles against spin, Root tends to throw it away quite often. In terms of run scoring consistency, only Smith and to an extent Williamson are head and shoulders ahead of him at this point. Pujara comes close.
The only thing that I would like him to work on now is his play against spin. In Pakistan's strong batting line up, he was probably the weakest against spin compared to YK, Misbah and Shafiq. I would like to see him emulate YK and Misbah in that regard - in their ability to milk spinners. If he is to become a run accumulator to Shafiq's stroke player, and since Pakistan are not really touring outside of Asia in the next 2 years, he has to learn how to milk the spinners, attack and dominate them to an extent and utilize them as his chief run scoring opportunity.
Pakistan's next series is likely against Sri Lanka this year. There is no YK/Misbah to take on Herath this time. He will have to do it.