Is polygamy mubah or recommended? What are the opinions of the 4 Sunni madhabs on this? Girls suffer a lot in polygamy. The first wife suffers a lot. What is the remedy for this in Islam?
It depends on the circumstances: polygamy can be obligatory, recommended, permitted, discouraged or forbidden. The factors which effect the ruling include whether a person fears falling into zina, how he judges his ability to administer justice, what he intends by the marriage (such as having children or following the sunnah) etc.
See e.g. Is it permitted to not get married? - the ruling on marriage itself varies depending on the circumstances of the person - and the same applies for multiple marriages.
If a person does not feel any need or desire for polygamy then some like the Shafi'is and Hanbalis have considered it recommended to remain monogamous as it lessens the risk of falling into haram i.e. injustice towards the wives. While others have adopted the position that it is recommended to practice polygamy in this case as it is following the sunnah.