Using the equation $\mu = \sqrt{n(n+2)}$, where $\mu$ is the magnetic moment and $n$ is the number of unpaired electrons, we can substitute $\mu = 4.90$:

$4.90 = \sqrt{n(n+2)}$

Squaring both sides of the equation:

$(4.90)^2 = n(n+2)$

$24 = n^2 + 2n$

Rearranging the equation:

$n^2 + 2n - 24 = 0$

Solving the quadratic equation, we find:

$n =4$

Therefore, the metal ion has 4 unpaired electrons.