4k at 60fps youtube videos on Safari is not possible. On Chrome it is.

Running v10.0.3 Yosemite and Safari 8.0.6 here and I can view the 4K/60FPS AERIALS video fine.

Try troubleshooting.

From your Safari menu bar click Safari > Preferences then select the Privacy tab.

Click: Remove All Website Data

Then delete the cache.

Open a Finder window. From the Finder menu bar click Go > Go to Folder

Type or copy paste the following

~/Library/Caches/com.apple.Safari/Cache.db

Click Go then move the Cache.db file to the Trash.

Quit and relaunch Safari to test.

If that didn't help, from the Safari menu bar click Safari > Preferences then select the Extensions tab. Turn that OFF, quit and relaunch Safari to test.

If that helped, turn one extension on then quit and relaunch Safari to test until you find the incompatible extension then click uninstall.

If it's not an extensions issue, try troubleshooting third party plug-ins.

Back to Safari > Preferences. This time select the Security tab. Deselect: Allow plug-ins. Quit and relaunch Safari to test.

If that made a difference, instructions for troubleshooting plugins here.

Try a restart.

Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.

Isolating an issue by using another user account

If the problem is still there, try booting into the Safe Mode using your normal account. Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

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Safe Mode - Yosemite

Hi, I have seen your post.. looks like that video it's not proper 4k.. maybe that persone who loaded 4k video not activated on youtube to upload HD video.. Cause facebook has that option in settings.. you have to activate it once, then it's allow you to upload HD videos and when you watch video you can choose on the bottom right corner (HD icon) if you want watch it HD quality or not, so I think it could be same with youtube.

I have downoaded your video.. and another 4K video to test it.. Your sample is not proper 4k.

when I open proper 4K video it comes on all my MacBook Pro Retina dislay.. when I open your video it's only covers three quarters of the screen.

You can see that your video resolution is only 1440p.. proper 4k is 2160p. Hope it's helps.

By the way.. I have activated my Macbook Pro Retina full screen resolution, now I got 2880x1800.