Act 4 is restricted to the last story mission of the Broken Mines dungeon (no Phase Beasts available) hence the Sundered Battlefield of Act_3 is required.
Here's the info needed for players:
https://torchlight.fandom.com/wiki/Avoid_the_Lava

Originally posted by https://torchlight.fandom.com/wiki/Avoid_the_Lava:

Avoid the Lava is a Phase Beast Challenge encountered in Sundered Battlefield. The dungeon consists of a system of metal platforms and pathways with deadly rising lava gradually flooding the area.

Found around the map are eight valves. Activating a valve temporarily halts the fiery torrent, buying time to open as many of the 18 chests (14 medium, 4 small) as possible, before the lava consumes all but the top platform (where one is always safe).

Wearing gear that offers faster movement speed and using a travel skill like the Embermage's Frost Phase are helpful.
As there are no monsters present, donning complete gold- and magic-finding farming gear is relatively safe (when opening a chest there is an occasional trap/Cursed Sword encounter to watch out for however).
In NG+, the following Skulls are dropped: Skull of Riechliu (+1540 Health), Rambren Skull (+60% bonus to Critical Damage), Penderal Skull, Qzmonda Skull, Skull of Quato, Skull of Samadow (+100% damage do secondary targets).