Here is a list of updates I found in 1.9 prerelease 4

Obviously this is not a complete list since it just came out.

Significant updates:

The End

Ender Portals

Ender Eyes are required to activate the portals

Endersand (currently has a cobblestone drop)

f5 now has 2 third person modes (front and back)

Round Sun and Moon

Moon Phases

New potion functionality

Smaller Updates:

Gold Nugget Melon(better name please)

Please comment if you find more and I will add them to this list!