The Tab S4 does not come with Samsung's Theme Store that you would find on their phones, so you've basically got 2 options.
A. Change the wallpaper. This can be done by installing a wallpaper specific app like Zedge, or you can do what I do and simply do a google search for what you're looking for and then save the image to your device.
or
B. Install a 3rd party launcher such as Nova, Apex, etc. You'll still be able to change the wall paper as you'd like, but will also have to option to install custom icon packs (available in Google Play) along with some custom widget options like Zooper (Zooper doesn't appear to be receiving anymore updates, but other than it's weather services, everything else about it still seems to be working just fine).