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What's if should be set higher than 1920 px + Retina image breakpoint for high quality content display for 4K resolution?

It's not common to set a breakpoint for larger resolutions.
Normally, the desktop is the largest view and you add breakpoints to adjust for smaller screens.

Thank you Pablo. But what about quality of media: pictures, videos etc?

You can use the best quality for the default view and if necessary use lower quality in breakpoints.

Thank you Pablo for suggestion

And last question, the default breakpoint should be bigger than 1920px - the last breakpoint?
Otherwise, it won'tb be visible.

The default view is not a breakpoint. But it should be wider than all breakpoints.

This is also mentioned in the FAQ:
http://www.wysiwygwebbuilder.com/forum/ ... hp?t=63817