You can now reserve a 2019 Cadillac XT4, at lest I think you can using the “Reserve Yours” link on vehicle’s landing page. You should be able to build your XT4 and then follow the prompts until you get to the page where you select the closest the preferred dealership and enter your info. I'm not in a rush, but if you want to be the first ones getting a Xt4 then you may want to reserve one soon.
I did the Reserve process on April 7, 2018 at my local dealer, and the same day provided my order info...fully loaded Sport. Took some time to get the order number from them, with the detail of the confirmation of options, including detailed pricing. That was dated May 4, 2018. Have been advised Cadillac will build pre-determined optioned XT4s for 6 weeks, likely starting in August 2018. Fully equipped units, such as mine, will not be built until after the initial 6 weeks, maybe even after another 6 week period. I'm thinking the vehicle will be built in October 2018 at the soonest.