Here's the complete list of changes from previous editions to the 4th edition (when the Arena rebranding happened).
- New Art on spellbooks, game board, rulebook, box, and several cards
- Game Board folds in a different way so as to fit in the new, more compact box
- Some new tokens added. Previously they just said to use whatever you had at hand for tokens (different from markers which were always provided) but they got a lot of complaints that tokens weren't included, so they added them. These include wrath tokens, rage tokens, melee +1 tokens, and several others.
- 14 additional cards. These are: +1 Dragonscale Hauberk, +1 Feral Bobcat, +1 Leather Boots, +1 Leather Gloves, +1 Minor Heal, +1 Regrowth Belt, +1 Teleport, +1 Thunderift Falcon, +2 Dispel, +2 Dissolve, +2 Force Push.
In addition, like every previous edition, all of the cards have the most recent errata baked into them at the time of the printing. You can find all errata for all of the cards in the Official Rules and Codex Supplement. (also, when I say all of the cards, very few have actually received errata).