The ratio of the velocities of light of wavelengths $4000\mathit{Å}$ and $8000\mathit{Å}$ in vacuum is $1:1$ because light travels with same speed in vaccum (i.e., $3\times {10}^8\mathrm{m}∕\mathrm{}\mathit{s}$ ) and it does not depend on wevelength or frequency.