Answer
$$7.0 \times 10^{3} \mathrm{m} / \mathrm{s}
$$
Work Step by Step
The root-mean-square speed given by See Eq. $19-34 .$ $$v_{\mathrm{ms}}=\sqrt{3 R T / M} .$$
The molar mass of hydrogen is $2.02 \times 10^{-3} \mathrm{kg} / \mathrm{mol},$ so
$$
v_{\text {rms }}=\sqrt{\frac{3(8.31 \mathrm{J} / \mathrm{mol} \cdot \mathrm{K})(4000 \mathrm{K})}{2.02 \times 10^{-3} \mathrm{kg} / \mathrm{mol}}}=7.0 \times 10^{3} \mathrm{m} / \mathrm{s}
$$
