The like terms are those which have the same power of r. So, we have $r^{5}$ and $3r^{5}$, $-r^{3}$ and $r^{3}$, $r$ and $2r$, and $-4r^{4}$ as it is.
So, the expression becomes:
$$\left(r^{5}+3r^{5}\right)+\left(-r^{3}+r^{3}\right)+\left(r+2r\right)-4r^{4}$$
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