Proof Writing & Problem Solving · Medium problem

Every cell of a $4 \times 4$ board starts with the number $0$. A move chooses a $2 \times 2$ block of cells and adds $1$ to each of its four numbers. After some moves the board reads, row by row (top to bottom), with one entry hidden:

$1, 3, 2, 0$

$4, x, 5, 2$

$3, 6, 6, 3$

$0, 2, 3, 1$

What is $x$?