Which of the following hydrated ion has higher ionic mobility than ${\mathrm{Be}}_{\left(\mathrm{aq}\right)}^{+2}$?

${\mathrm{Li}}_{\left(\mathrm{aq}\right)}^{+},{\mathrm{Na}}_{\left(\mathrm{aq}\right)}^{+},{\mathrm{Mg}}_{\left(\mathrm{aq}\right)}^{+2},{\mathrm{Ca}}_{\left(\text{op }\right)}^{+2}$

Larger the size of the cation, lower will be its hydration, which leads to a smaller hydrated size and a higher mobility. Here, all the given ions has higher ionic mobility than ${\mathrm{Be}}_{\left(\mathrm{aq}\right)}^{+2}$.

Thus, the correct answer is 4.