Yes you are good to go with the 4K. If some reason the 4K did not have Atmos/Dts:X then nothing else will either.
Netflix, in particular, but also Prime has Atmos content, however this is really just DD+ with Atmos metadata. DD+ is a compressed audio format, so only real discs will have the uncompressed versions. The streaming services do not currently offer Dts:X in any form. I have tried Netflix and the Atmos it does work on a PC, xbox one, Nvidia Shield, Apple Tv and possibly the top Roku. Some current TVs, (2017-2018) will support DD+ and Atmos via Netflix, over Arc, but you would need to check the particular model. Once eARC becomes a thing, no current Tvs have this yet, although most of the new AVRs from Denon, Marantz and Yamaha do, then things may improve further.
There are a number of services which sell or rent movies, Apple Store, Google Movies, Prime and a few others. I have not used them so I cannot truly comment on the quality for either picture or sound. Others maybe able to help here. However if it is the same as the Netflix/Prime then the quality is ok and sound can work fine.