There's just too much misinformation flying about and it's making everyone crazy. I partially blame that (incredibly misinformed) Tumblr post about Seasons 3's sales making everyone think that Crystal's future hinged on the success of its bluray sales in Japan despite the FAQ of the source that actually said this:
1.1 How important are DVD/BD sales?
It depends! Great disc sales can make a show, but low sales won’t necessarily break it. A detailed explanation of “how the business works” requires a lot more detail but for now it’s sufficient to know that DVD/BD purchases are only one revenue stream sharing the burden with potentially many others...
The Sailor Moon franchise is still doing incredibly well as a whole. As of 2016
, it is the fourth major earner for Toei in Domestic Licensing (341 Million Yen) and Overseas Film (318 Million Yen) in the fiscal year of 2016.
Of course, it's hard to distinguish how much of that is Crystal or Classic, but the anniversary has been going on since 2012, and Sailor Moon didn't start making 200-300 million yen in different categories until 2014-2015 when Crystal first started. Regardless, none of Season 1, 2 or 3's bluray sales would even account for that much money since no volume has sold more than 5,000 copies, so to panic over BD sales is naive.
The reason why Season 4 hasn't been announced yet is simple... They're probably not ready. With both Kon working on Naruto and Takahashi busy with Aikatsu; Toei has to once again find new a new director and CD during a year which at a glance looks like their BUSIEST in a while.
Not only do they have find new staff though, as with any anime, they have to assemble a production committee, sponsors, etc. There's a lot of puzzle pieces that have fall together in order to make an anime...
I say just be patient and wait a few more months before panicking. If it seems like the Sailor Moon gravy train is still slowing down by Spring, then yes I'd be concerned.