You must put one of the cards into your hand, even if you’d rather put all four on the bottom of your library.

If you control Lone Revenant and any other creature (including another Lone Revenant), its ability won’t trigger.

Check whether Lone Revenant’s ability triggers after combat damage is dealt but before any creatures that are destroyed due to combat damage leave the battlefield. For example, if you control Lone Revenant and another creature, and that other creature takes lethal damage in the same combat damage step that Lone Revenant deals damage to a player, Lone Revenant’s ability won’t trigger.

If you control no creatures when Lone Revenant’s ability resolves, you’ll still look at the top four cards and put one into your hand.