Quite a few people have given adequate answers but there are a few things that must be corrected so...
In Islam a man is allowed to have four wives with whom he has to be utterly and completely just and give equality in terms of finance, lodging and time. there can be no compromise regarding this because that would then be a grave
sin.
The man need not take the permission of his other wives as per the rules set by Islam but some countries have adopted this law seeking to protect the rights of women because mostly men these days would not give the previous wives their due right.
However, the right law would have been to empower the women to be able to go ask the government to intervene on her behalf and get her the rights she deserves. Sadly, the huge and small democratic governments, all have failed miserably to provide basic rights and protection to its citizens. oppressors prosper! Keep the recent Delhi gang rape case in mind, as in how in the middle of the city on a moving bus the crime was committed with the oppressors having no qualms about carrying out the heinous crime. We must think of what gave them that confidence that they could do that and throw the two battered people on road and go away and go on with their lives normally!
Also, the reason behind second marriage need not be infertility, it may just be liking some other person as well. If the man is happy with his wife that way he will continue to live happily. I know this will be hard to understand but it is this way. This is one of the things that sparks off huge debates but for that people need to see it with the perspective of a how Allah (SWT) has created us with the purpose defined. I am just stating it here to correct the info given, however, no amount of debate will clarify it :) for we all have our own mindsets on lives, the way we see it. So peace :) Also keep in mind that it is allowed and not obligatory. A right kind of Muslim will think a hundred times before doing it because the consequences of not fulfilling the conditions are too severe. Peace :)❤️