I know it's tough getting started, but hang in there. Some suggestions for the 4th grade teachers:
If choice time is difficult to manage they can treat each activity separately, providing they have enough materials. Teach the game first so the kids have something to do if they finish the activity early.
The related problem sets in Landmarks are much more challenging than those in Arrays and Shares. You may try some of those first. The "relatedness" of the problems is more obvious.
The are video tapes that may be available from TERC of teachers conducting "sharing" sessions. These are great models of how to manage classroom discussions, what questions to ask to focus the kids' thinking, etc. If tapes aren't available, you could try to visit a classroom of an experienced Investigations teacher or have a teacher visit you.
Finally, you can't master everything at once. This kind of change takes time. For what it's worth, I felt just like your teachers probably feel when I first started using Investigations. Now, in my 5th year, the program feels really comfortable and I can hardly believe what the kids are able to do in math.