I had noticed the Gears of War 4 product page was running on the Xbox One marketplace so I decided to take a look around and see what information was available. Aside from the E3 2015 video and some screenshots from there I also found that an add-on was already available. Upon further viewing it would seem at this point that some of the gun skins for Gears of War: Ultimate Edition with be compatible with Gears of War 4. I included some screenshots below so you can see the various views of the pages and how they're connected. It's a neat viewing and I wonder if character skins and other things like maps will also be available for Gears of War 4. The first image below is the actual page and you can see if you scroll to the right that the "Animated Weapon Skin Pack 2" shows up as an add-on.
Once you click the skin pack to view you see this information pop up and it looks like it's bundled with the Gears of War Ultimate collection. Though if you scroll over a bit more there's a compatibility section which you can click on.
That brings up this image below with the all the games it's compatible with. It clearly shows the pre-order for Gear Ultimate, two regular Gears Ultimate titles and Gears of War 4. It'll be interesting to see if this is true or just an issue with the system currently. Just a neat bit of information in case you were considering missing out on certain versions for Gears of War Ultimate and want fancy items in the new game.