Berakhot 27

What does it mean "until four hours" - until and not including the fourth hour or until and including? Who do we hold like regarding times for the morning prayer and for the afternoon prayer? Can one pray arvit, the nighttime prayer, on Friday afternoon before sunset? And if one does this, is it considered Shabbat for them? If they accidentally accepted Shabbat early, thinking it was nighttime (i.e. gray rainy day), when they realize their mistake, is it Shabbat for them? Is arvit obligatory or optional? Rabban Gamliel and Rabbi Yehoshua disagree regarding this and Rabban Gamliel embarassed Rabbi Yehoshua in the biet midrash because of their disagreement. The people got angry at Rabban Gamliel and removed him from his position of being head of the beit midrash. There was a discussion about whether to replace him with Rabbi Yehoshua, Rabbi Akiva or Rabbi Elazar ben Azaria. The latter was chosen and he went to colsult with his wife about whether to take the job.