Mini display port to Hdmi 4k 60Hz.

@KurtLoki  Most MiniDP to HDMI adapters are passive and therefore rely on the source MiniDP port supporting "Dual Mode DisplayPort", which allows the port to switch to native HDMI signaling.  But even if your MiniDP port supports 4K 60 Hz when sending native DisplayPort, it might not support that when running HDMI signaling.  That would require HDMI 2.0 support, but that port's Dual Mode DisplayPort interface might only support HDMI 1.4, which would only allow up to QHD 60 Hz or 4K 30 Hz.  In that case, even if the adapter would support 4K 60 Hz, the problem is that the source doesn't.  The solution to this is to get an active MiniDP to HDMI 2.0 adapter.  In that case, the system is sending a native DisplayPort signal to the adapter rather than falling back to Dual Mode DisplayPort, and then electronics within the adapter itself switch that to HDMI 2.0.  If you're in the US, here is an example of what you'd need, made by Cable Matters, which has a solid reputation for quality products.  I've personally bought several of their cables and adapters and they've never let me down.

@jphughan Thanks for your reply. There is a news about it. I've tried to connect my laptop to my Sony Tv that support 4k 60Hz (even more) by direct HDMI port and Minidisplay port with adapter certified to HDMI. As already said, with the same cable, same resolution, and same TV HDMI port it works only by direct port HDMI from laptop. Now i've been to my office and i've tried to use the same adapter Minidisplayport/HDMI to my LG monitor 4K 60Hz. Voilà it works fine at 4K 60hz, even with both monitor (2 identical monitor LG) connected. Do you know how it possibile?

Thanks

@KurtLoki  If the same laptop output, same adapter, and same cable gave you 4K 60 Hz in the office and only 4K 30 Hz at home, then I'm not sure how to account for that.  It could just be one of those strange interoperability problems that shouldn't exist in the tech world but does anyway.  There are all sorts of them.  For example, I've helped multiple people here who said they had a USB-C to HDMI adapter that didn't work with their Dell laptop even though it worked fine with another device they tested it with.  After basic troubleshooting didn't resolve the issue, I said, "Well it's either a problem with your USB-C port in general, which would require a motherboard replacement [they didn't have another way to test USB-C video output], or else it's a strange interoperability issue that affects only the specific combination of that system and that adapter.  Since a motherboard replacement would be a huge ordeal, you might want to try a different brand of USB-C to HDMI adapter to see if it works with both your Dell laptop and the other device you're testing with."  In those cases, a different USB-C to HDMI adapter resolved the issue.  That's happened 5 times now.  Obviously that shouldn't have been necessary because both adapters should have worked with all systems, but not everything in the tech world that SHOULD work actually DOES work, unfortunately.