XO-4b is an extrasolar planet approximately 956 light years away in the constellation of Lynx.

It takes only 4.125 days (or 99 hours) to orbit at a distance of 8.3 gigameters (0.0555 AU) away from the star.

[1] The study in 2012, utilizing a Rossiter–McLaughlin effect, have determined the planetary orbit is strongly misaligned with the equatorial plane of the star, misalignment equal to -46.7±8.1°.

The name was selected in the NameExoWorlds campaign by Estonia, during the 100th anniversary of the IAU.

Hämarik is Estonian for dusk, and was named for a character in a folk tale written by Friedrich Robert Faehlmann.