4. If all positive integers are written in ascending order in a row, the 2016th digit from left to right is $\qquad$
There are 9 single-digit numbers, 90 two-digit numbers, and $x$ three-digit numbers, then $9+180+3 x \leqslant 2016 \Rightarrow x \leqslant 609$, and the 609th three-digit number is 708, so the 2016th digit is 8.