It looks like this is kind of my problem too - http://www.polycount.com/forum/showthread.php?t=89667
Partial solution is to double every layers with translucenc lit material and put them right behind and change their material to for example maksedlitspritematerial. DoF region works but still there are some glitches and borders from alpha are not smooth. Pre-blur textures in PS is unfortunately not solution for me.
If DoF is not compatible with translucent materials, is there any way, how apply/animate/put blur effect on texture directly in UE4?