7. The sum of the first $m$ terms of the arithmetic sequence $\left\{a_{n}\right\}$ is 90, and the sum of the first $2 m$ terms is 360. Then the sum of the first $4 m$ terms is $\qquad$ .
$=\mathbf{7 . 1 4 4 0}$.
Let $S_{k}=a_{1}+a_{2}+\cdots+a_{k}$, it is easy to know that $S_{m} 、 S_{2 m}-S_{m}$ 、 $S_{3 m}-S_{2 m}$ form an arithmetic sequence. Therefore, $S_{3 m}=810$.

It is also easy to know that $S_{2 m}-S_{m} 、 S_{3 m}-S_{2 m} 、 S_{4 m}-S_{3 m}$ form an arithmetic sequence, then $S_{4 m}=3 S_{3 m}-3 S_{2 m}+S_{m}=1440$.