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$\text{If I understand what you're saying...}\\ 4\sqrt{abc}=5\\ 4\sqrt{abcd}=3\sqrt{10}$

$\dfrac{4\sqrt{abcd}}{4\sqrt{abd}}=\sqrt{d} = \dfrac{3\sqrt{10}}{5}\\ d = \left(\dfrac{3\sqrt{10}}{5}\right)^2 = \dfrac{9\cdot 10}{25} = \dfrac{18}{5}$