Can I run 4K at 60Hz on an external display and 60Hz on the built-in display on a 2015 MacBook Pro?

Hi,

Following document could help your question.

https://support.apple.com/en-us/HT206587

Use of an external display will to affect the refresh rate on the Internal display, and vice-versa. Each has independent hardware to support it at full resolution and high refresh rates. DisplayPort family and ThunderBolt provide fastest refresh rates.

Do not make the mistake of assuming that "Legacy" display interfaces can do as well as DisplayPort, especially at higher resolutions. Also, a direct adapter may be required. Many Docking stations bypass the built-in Hardware support, and could produce remarkably poorer results.

<<I don't want to offend you but your replies don't seem to be relevant.>>

I am glad you have already found the solution for you situation.

But this forum and this discussion will be here for YEARS, and users who are not as expert as you may find it by searching. It is therefore my practice to enumerate some of the most prominent pitfalls of the subject under discussion before I walk away.

Please feel free to ignore anything that you do not find relevant to YOUR situation. Others who read this discussion later may find it very relevant.

Running an external display does not disable the Internal display. You can run each separately.

If you want the same picture on both, they need to be set to the same resolution.