Saxat från rpg.net:
4th edition-
Base starting time period is the height of Arthur's Reign.
Includes a numerous cultures and religions for starting cultures.
Includes rules for magic and spell-casting characters.
5th edition-
Base starting time period is during Uther's reign.
Only gives rules for Christian Cymric knights from Salisbury.
No magic rules.
The advantage of 4th is that you get a LOT more options, but the organization is lacking.
5th loses the options and magic, but does really tighten the focus of the game and is much better organized.
Both are solid books in their own rights and I personally use the crap out of both books. (I'm one of those who likes the 4th ed magic system.)