Would it though? I personally don't see any hardware area where the S4 critically needs an update. Yes, a larger battery would be nice, but the S4 SoC seems to have all the power it needs for the moment and the new screen sizes seem to be generally well-received.
Jason Snell just posted an opinion piece on Macworld noting he feels Apple does not need to release a Series 5 watch next month and I am in agreement with him.
I certainly am not in the market to replace my Series 4 and I would not be surprised if most felt the same. I understand those on an earlier Series who skipped S4 would prefer to go to an S5 rather than get an S4 now and have "lost" a year should a fundamentally better S5 come out in Late 2020 (which I kind of don't see happening - I see the S5 being a refinement of the S4 as the S3 was to the S2).
Offering a ceramic Watch Edition S4 would offer an upgrade path for those with S2 or S3 Watch Editions who are now multiple years on their current watch, so that makes sense. And titanium might appeal to those on earlier Series who would like something more durable than aluminum, but do not like the weight of stainless steel.