Timeline:
December 2001 - Pikmin releases on Gamecube.
August 2004 - Pikmin 2 releases on Gamecube
March 2009 - Pikmin NPC releases on Wii
June 2012 - Pikmin 2 NPC releases on Wii
July 2013 - Pikmin 3 releases on Wii-U after being in development for Wii.
July 2015 - Eurogamer interview - Shigeru Miyamoto states "The next pikmin is very close to completion. Pikmin teams are always working on the next one".
July 2016 - Gamerant Interview - Shigeru Miyamoto states "You know, when we're in development we have to create a list of priorities and it has been hard to kind of fit [Pikmin 4] into that list," he said. "But we're hopefully starting to see that on the list now "
March 2017 - Nintendo Switch released.
April 2017 - Hey Pikmin! Announced. Developed by Arzest (Not an internal Nintendo team)
June 2017 - Eurogamer interview - Shigeru Miyamoto states "Pikmin 4 still progressing" seemingly confirming Hey Pikmin was not "Pikmin 4" from the Miyamoto interviews.
July 2017 - Hey Pikmin! Spinoff released.
Feb. 2019 - Pikmin 3 port for switch "Rumored" to be in development by certain high quality insiders. From those same insiders that Pikmin 4 might have been "rebooted" some time ago.
Another E3, Another Nintendo Direct, No signs of life from the Pikmin franchise. So what can we theorize from the above timeline? Nintendo and Miyamoto mysteriously quit talking about Pikmin 4 after June 2017. We can't blame this on the Switch because they would have switched (lol) development well before 2017. So we can assume everything was going to plan through the June 2017 interview. Sometime afterward, Pikmin 4 hit a wall. According to a highly respected insider, Pikmin 4 most likely was rebooted sometime in 2017. Is 1.5-2 years of development not enough to warrant an announcement video? Metriod Prime 4 was announced extremely early in development at E3 2018 without any CG or gameplay footage just to assure fans. Why can't they do the same for Pikmin 4?
The case is curious indeed. I wonder what our next chapter will be. What do you think is happening to Pikmin 4?
Credit for Pikmin 4 boxart: KonbiniReddit