  If $$ \Delta(x) := \sum_{n\le x}c_n - Cx $$ denotes the error term in the
classical Rankin-Selberg problem, then it is proved that $$ \int_0^X
\Delta^4(x)\d x \ll_\epsilon X^{3+\epsilon},\quad \int_0^X \Delta_1^4(x)\d x
\ll_\epsilon X^{11/2+\epsilon}, $$ where $\Delta_1(x) = \int_0^x\Delta(u) du$.
The latter bound is, up to `$\epsilon$', best possible.
