i follow the fx line obsessively and did noticed the fxs4 was not available on their site but the mens was... interesting.
from what i've learned ordering treks in the last few years, go to your dealer and ask them to check warehouse inventory. this will tell you if they have the 2018 fxs4 or not. it's quite possible that the website simply isnt updated yet. same goes the other way - if you dont see an older bike listed on their site anymore, it's possible your local warehouse (or even local bike shop) may still have that model.