Act 4 or Act IV: The Harrowing in Diablo 2 is the fourth Act and can be accessed by completing The Guardian quest from Act 3. The player will begin the Act in Pandemonium Fortress, which is the last outpost of Heaven at the gates of Hell.
Locations where a waypoint can be found will be marked below with this waypoint icon: 
Act IV: The Harrowing
Town
Surface Areas
Dungeons
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River of flame has a waypoint.
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