Hey all, just thought I'd post what I gathered were rule changes. Let me know if there are errors/you find more.

Determination Tokens

• You can now keep and spend as many determination tokens as you want to add +1 per spent token to any attribute test. ( 3rd Ed: You accumulated tokens until you passed the test, then lost them all.)

Encountering Monsters

• You now skip encountering the dungeon chamber if there is a monster token. (3rd Ed: You had to fight the monster, then encounter the chamber on the same turn.)
• You may now attempt to escape a monster at the end of each combat round. (3rd Ed: You could only escape before the first combat round.)
• Escaping now requires only passing an agility test, regardless of monster type. (3rd Ed: Escaping required each combatant to draw a random card from their unique power deck and compare numbers.)
• You now may escape through portcullises, ignoring them. When you escape, you may now escape to any explored chamber without encountering it. Next turn, you may move or search as normal. (3rd Ed: You could not escape through a portcullis, and you must return to the room you came from. On your next turn, you had to encounter that room without moving.)
• Monsters are now always drawn randomly, including when encountering monster tokens. (3rd Ed: The monster type was always specified and there were tokens for each monster type.)
• Combat is now resolved via card-based rock, paper, scissors, with no "power decks." (3rd Ed: Card-based combat system with counterattacks, deathblows, etc. and unique power decks for each hero and monster.)

Catacombs

• When exiting the catacombs, you now suffer one wound for each extra space beyond a wall the die roll would take you. (3rd Ed: You suffered no wounds this way.)
• When exiting the catacombs, you immediately encounter the dungeon chamber or monster token. (3rd Ed: As with escaping, when you exited the catacombs, you immediately ended your turn, but then you had to encounter your chamber without moving on your next turn.)
• A passage down to the Catacombs may now appear on a Cave-In space. (3rd Ed: You may immediately move to the Catacombs via a Passage Down card, but could not place the marker on a Cave-In space.)
• The catacomb entrance markers are now intended to be unlimited. (3rd Ed: They were limited to 6 tokens.)

Moving and Encountering Dungeon Chambers

• You ignore doors and portcullises when moving from Corridors or Chambers of Darkness. ( 3rd Ed: You could not ignore doors or portcullises from these spaces; you only ignored a portcullis upon first entering an unexplored chamber with one at the entrance arrow.)
• If your path out of the Chamber of Darkness is blocked, you reroll. (3rd Ed: You ended your turn.)
• When encountering a Spider Web chamber, you immediately move again, regardless of whether you retreat or go forward crossing the Web. ( 3rd Ed: You could only move again immediately if you went forward; if you retreated, you had to wait until next turn.)
• When entering a tower, you may now teleport to any other tower room and move again. ( 3rd Ed: You could not.)
• To exit the dungeon, you have to wait until the start of your action phase on a tower space. (3rd Ed: You could move onto a tower and immediately leave.)
• If there are multiple portcullises in a hall, you must now pass both of them to cross. (3rd Ed. You only had to test Strength once to open both portcullises.)
• Chambers can now only be searched once per game. (3rd Ed: They could only be searched twice before a character moved, but infinitely during the game in total.)

Two (maybe 3) additional changes I've noticed;

* When you enter the Catacombs your turn ends. ( 3rd Ed: You draw a catacomb card when you enter ). ?

* The Sun Token advances on the First turn of the game.

* You do not move again after a Bottomless Pit.