We obtain some new upper bounds on the maximum number $f(n)$ of edges in $n$-vertex graphs without containing cycles of length four. This leads to an asymptotically optimal bound on $f(n)$ for a broad range of integers $n$ as well as a disproof of a conjecture of Erdős from 1970s which asserts that $f(n)=\frac12 n^{3/2}+\frac14 n+o(n)$.