I agree that chances seem very low for Smash Bros 4 trailer to be shown at E3. Sakurai may be a workaholic but he isn't a magician. Smash Brothers Brawl was announced at pre-E3 2005 and development didn't start until October. The first trailer was revealed in May 2006. Kid Icarus: Uprising's development was completed in late January. In addition, Sakurai still hadn't formed a complete team by then (assuming he even started). By late February, Sakurai said that he had begun brainstorming ideas by reflecting on what he had done with SSBM.
Come to think of it, Nintendo said that it will be attending the Nordic Games Conference event at the end of this month with plans to meet third-parties interested in "publishing opportunities" for the 3DS and Wii U. Perhaps Nintendo also intends to search for some devs that are available to help Sakurai work on Smash Bros 4. We know Game Arts was involved in the main part of the preliminary development of Brawl and Monolith Soft contributed to the project as well.
Come to think of it, Nintendo said that it will be attending the Nordic Games Conference event at the end of this month with plans to meet third-parties interested in "publishing opportunities" for the 3DS and Wii U. Perhaps Nintendo also intends to search for some devs that are available to help Sakurai work on Smash Bros 4. We know Game Arts was involved in the main part of the preliminary development of Brawl and Monolith Soft contributed to the project as well.