I assume that by "4th edition" you mean "revised 4th edition" as it's the most recent version and differs substantially from its non-revised predecessor.
Talisman was always a very random game with many effects depending on the die roll.
To somewhat counteract this, revised 4th edition introduced the Fate stat.
Spending a Fate point allows you to reroll 1 die, giving players some amount of control over the fate of their character.
In my opinion, this is the most important difference.
Many small changes address the balance issues from previous editions.*
Last but not least, the game finally** is more formalized.
For example, in the 2nd edition an effect might refer to a weapon, and the players needed to figure out on their own, whether a given piece of equipment is a weapon or not.
In the most recent edition if an item is a Weapon it is explicitly stated on the card.
*although 4th edition has it's own brand-new balance problems.
**to be honest, this might be already the case in the 3rd edition - my knowledge about that version is very small