Streaming 4K content from iMac to Apple TV

I bought a 4K Apple TV 64 GB, model 2017 (with the black remote)
I want to stream 4K homevideo's to it from my iMac.
Point is that it takes a long time to get the content to the Apple TV.
iMac is wired and ATV is connected through WiFi.
I do not understand why it takes so long since streaming 4K content from YouTube 
is running (almost) immediately. Why isn't this the case from a computer in the same house?
ATV has a 5 dot connection to WiFi and as said the iMac is connected to router bij cable.

THX guys!

Because I know absolutely nothing about this let me offer what limited education I have on this subject can surmise.
YouTube starts streaming immediately by sending packets of data and updates video content as needed. Like running a marathon and taking on water at an aid station every half mile instead of starting the race running with the entire amount needed in total. You're running carrying a gallon in each hand. Hardly efficient.
I can't stream native 1080 ProRes (encode to h264 for it to play reasonably well) to the lobby over a wired connection and you're trying to punt 4K over WiFi. I'm thinking all the data (file size, video length, bit rate, fps, codec, etc) needs to load first before the player begins playing it. That's how my primitive brain understands these things.

Hi VTC. Thanks for your reply on this matter. I think you are right about how YT does this, must be someting like that. Big question of course is why a giant like Apple does not use this technology? This morning i again tried to play around 3 minutes of downloaded 4K content what i exported from FCP in 4K and copied into Apple TV (the app in my iMac, not the mediaplayer, very confusing to use same name for a mediaplayer AND an App). This morning i could play it biut i do not know how long it took to download it from the iMac. 

I stream HD and 4K to my AppleTV from my MBP via Wifi/AirPlay just fine. I've been playing HD content from Macs to AppleTVs successfully with no issues for years both at home and in commercial applications.

Hi Ben. So, what you are saying is that it must be the WiFi or ethernet in my case? I did not try airplay since my iMac is upstairs. In case of a MBP airplay is better possible of course. maybe Airplay does transport the content in packages, i don't know. I assume you play the content on your MBP and stream it to the ATV? I play it on the ATV and it must be downloaded from the Mac, that might be the big difference.

I'm not exactly sure how you are streaming to the ATV if you are not using airplay, but I concur with Ben. We use airplay from iMacs, Macbook Pros and iPads a lot to 4K HDR TV's and they always work well. Select a video on your computer, select the airplay icon and link to your ATV. This should work fine.

AirPlay was designed for video/audio streaming between devices.

As i said i do not use airplay in this case since the iMac is on another floor. I use homesharing, that is what it is called i believe. I just drag the video into iTunes (is now the APP Apple TV) and this content can then be found on my DEVICE Apple TV (downstairs in living room connected to TV set or beamer) When i click on a video to play it then it starts downloading the content from the Apple TV APP tot the Apple TV DEVICE. So it is noet streaming as i said before, it is download and play. Or maybe a mix of both, i do not know exactly. But playing content from YouTube starts immediately and my own content takes long te start. I assume that is because YT sends in small packages and Apple TV APP total file in one big package. Question is if there is any way to make it work like playin YT content. I will also look into Airplay in combination with REMOTE.

No, that is not a streaming function, it does in fact have to copy the file to your AppleTV. But once done, should play fine.

So, you need to look at your Wifi. How robust is it?

Yes, correct. Once it is there is plays fine. Problem then is watching another homevideo takes again a lot of time. I will check the WiFi also. Thx Ben!