Mother 4 plays like any other Mother game, or most RPGs in general. Some changes from the previous installments are that there are more than four party members, but you choose which to use in battle. There are 10 main party members, but two aren't required to beat the game. Also, characters can also attack using weapons (ex.baseball bats) instead of it being just a stat-raising item.
During the events of Mother 3, a Pigmask built an 11th Porky Robot in case if Porky somehow cant lead anymore. The Pigmasks decide to take another attempt at enslaving humanity and use the 11th Porky Robot as their leader. Then, the Pigmasks move to a region called Zeru,where the main protagonist lives. He hears about the Pigmask's plan, and him, his father, his sister, his mother, and his cat(optional) who all have PK powers (exept for the cat, he has different attacks) decide to go on a quest to stop them.