It still isn't instantaneous but I had DROIDX , DROIDPro, and original DROID and DROID 4 shutter speed is way faster probably at least twice as fast if not 3 times as fast which is nice cause I take lots of pics of my kids. Its also nice to have shortcut straight from lock screen too
This is because the droid 4 has no 2 level shutter button, so when you tap the onscreen shutter it first has to focus and then take the photo. Most cameras and previous droids had the 2 level shutter, so you could half press to focus and then fully press to take the picture.
I can confirm on the ICS leak that it has tap to focus, so the shutter is more or less instantinous as long as you have focus.