Is it obligatory to sell one of the four she-camels if one of them is about to give birth?

Zakat is obligatory on camels if they reach the nisab (threshold) and a hawl (full lunar year) passes over them. It is permissible to sell any number of them if they are below the nisab. If the camels reach the nisab and you wish to sell some of them before the hawl passes, there is no harm in that, as long as the intention is not to use it as a trick or to evade Zakat. Indeed, "actions are judged by their intentions."