Plex (Free) - This is one of the most popular apps to stream media from your PC to your aTV4. You set up a Plex Media Server on your PC (any OS) and add folders for your movies, tv shows, music, etc. and it will automatically organize your media based on the file names. Then you can simply download Plex on whatever device you want to stream the media to (iOS apps are also available) and browse your library.
Pluto TV (Free) - Pluto TV is a free online television service broadcasting over 100 live channels full of TV shows, movies and Internet videos hand-picked by people who love entertainment. No searching. Turn on, lean back and flip through the familiar guide to enjoy late night, news, sports, classic toons, music videos and even cats. There’s definitely a channel for you on Pluto TV.
Crossy Road (Free) - This is one of the games which was showcased at the keynote where the aTV4 was unveiled. A very simple game where you control an animal and need to cross as many roads as possible. The real fun starts when a friend joins (iOS version is free as well). Then, you can revive each other but also block each other.
It can be a bit tricky at first to get into multiplayer mode. Launch the game on your aTV4 and on your iOS device. Press the up-arrow in the bottom right corner on your iOS device and then choose the controller-icon. On your aTV4, then confirm that you want to do multiplayer mode.
Alto's Adventure (currently $0.99 instead of $2.99) - A gorgeous and very simple snowboarding game which is even more stunning on a large TV than on an iDevice. Buy one one device and get it for all!
I'm not sure if this would help, but here is what I bought when I ran out of HDMI slots.
It has 3 HDMI slots and for me at least, when I turn power on to my ATV4 that is what displays, if I then turn on my PS3 it will switch to that automatically. I have my cable box on a separate HDMI slot, so I don't know how it would work with that.