Some photos are not syncing to iPhone 4
Deleting the Photo Cache may do the trick. 

To do so, just:


From the Finder, choose Home from the Go menu.
Once there, open the Pictures folder and locate the iPhoto Library
file. (For Aperture, this is called Aperture Library.)  If you have
moved your iPhoto Library or Aperture Library to a different
location, go to that location in Finder first.
Control-click the iPhoto Library or Aperture Library file and choose
Show Package Contents from the shortcut menu.
In the next window, locate the iPod Photo Cache folder and drag it
to the Trash. Important: Do not remove or modify any other file
within the library package. Removing or modifying other files may
cause data loss. Close the Finder window.


Extracted from iTunes: Understanding the iPod Photo Cache folder.