I am using MacOS Sierra and am trying to download the ProRes444 Codec in order to export my Adobe After Effects 4k RED footage in this format (probably using Adobe Media Encoder based on what I have seen others do). I found a download of 'pro video formats' that supposedly includes this codec, and downloaded a trial of final cut pro so that it could run ProRes444 on my computer, but am not sure how to find it in Media Encoder now. Are there other steps I'm missing? Thanks!
I believe you mean ProRes4444.
As I understand the general situation, and as you also imply in your question, one needs to install FCP in order to encode with the indispensible ProRes codecs in other apps (such as Adobe ones.) That's certainly what I've gotten used to doing on brand new machines. I'm not sure if the trial version of FCP would do the trick, but I don't see why not. I don't think there's a stand-alone installer of the ProRes codecs, at least not for encoding. It's probably just for decoding (i.e., playback of a pre-existing ProRes video.) Someone correct me if I'm wrong here.
Okay, so let's assume the ProRes codecs are successfully installed (for encoding, not merely decoding) on your machine. In Media Encoder, you can export using Presets, or you can export using custom settings. You want the latter. Check out this link to see the various ways to access the custom Export Settings. (I usually just click on the pre-assigned preset name to open the custom Export Settings dialog window.)
Once you get the custom Export Settings window open, you'll need to change the Format option to ProRes4444. If you don't see any ProRes formats in that dropdown list, they may not be installed properly yet. You can also check to see if the codecs are currently installed for encoding from AE without firing up Media Encoder. Just add an item to the render queue and try to customize the output format. If you don't see any ProRes codecs available in AE, you probably won't see any in Media Encoder.