Apparently they had originally planned for The Sims 4 to be always online, similar to the recent Sim City, however after massive outcry on top of it's general failure in sales, they scrapped the online component.
Plus, EA recently laid off a TON of staff members who were working on TS4. As far as I know, they've only brought one of them back. Details about TS4 were supposed to be released way back as soon as November, and yet here we are in March with no new information.
I think they saw how negatively people reacted to the trailer and the general appearance of The SIms that they went back to the drawing board. I'll be surprised if TS4 actually does come out in September like they had announced last year.