Probably meant to emphasize the 4 instead of h265 (since the parent they responded to mentioned h265 not h264) but used markdown instead of HN formatting so instead of 4 got _4_.

Taking part in the campaign to make Hacker News recognise underscores as the way to do italics. That's what you would do on a typewriter to indicate to the typesetter that something should be set in italics.

Just emphasizing the 4.

In markdown that would put the 4 in italics, where it's not supposed it still works as emphasis, but can be a bit confusing.

Pseudo-formatting for emphasis. If the page was markdown it would be bold/italicized.