$4 \mathrm{g}$ of copper gave $5 \mathrm{g}$ of its oxide means,

$1 \mathrm{g}$ of oxygen combine with $4 \mathrm{g}$ of copper.

$\because$ Eq. wt of oxygen $=8$,

Therefore, 

$8 \mathrm{g}$ of oxygen combine with $=4\times 8 \mathrm{g}$ of copper $=32 \mathrm{g}$

Hence, 

Equivalent weight of copper $=32$