First of all it is permissible as is stated in the Quran. Hence let us just accept it at that and that the prophet also had many. But with his first, she was the only wife he had until her death.
Why Allah made it permissible, He only knows the full reasons why.
Let us take a look at what was happening at that time. Many males were killed as a result of wars and what happened to those families that lost their fathers/husbands/providers/protectors? Having 4 wives meant that those males who could 'afford' it could take in the families that had lost the father by marrying the widower. Nowadays, this permission has been abused.
Interestingly though, I was watching an Imran Hossein video on Youtube and he mentioned doing a seminar in Malaysia and that in the feedback he came to know that those field workers working on the telecommunications towers all had only baby girls. They had no sons. It became a phenomenon. It seems our exposure to the radiowaves has an effect on the male chromosomes. Perhaps, Allah knows what is going to happen and that soon, we will be having a disproportionately large female population and not very many males.
Imagine how the competition to get husbands will be then? Perhaps to the point of stealing husbands from other wives? Allahualam.
On your question, how could a female have more than one husband? She can only have a child a year at maximum. With 4 husbands, how long will they have to wait before each can have their own? Even then, how would they know to whom the child belongs to? With 4 husbands she will most probably be spending most of her life pregnant.
eace:
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