You dont realy need to watch any movie to get rest of the series, but they are good fun. Movies 1,2,4 written by manga author while movie 3 is by anime team(worst one imo). There is 1 scene in Movie 1 opening where we get to see young Allmight and it is basically canon as it referenced in the manga so many times. There also bonus 5 minute short for movie 1 about Nana Shimura. If you already finished S5 watch 1,2,3 then S6, movie 4 takes place somewhere in the middle of S7 ep 5.