Ok, sorry to ask again but do you have any official links that clarify this?
I searched for a FAQ but no luck.
What I could only find is this:
D&D XP 2010: Dungeons & Dragons Essentials : Critical Hits
Interesting quote was:
Also they talk about solo mode and a different way to determine your class or role. Perhaps they have designed classes in a way that can perform on more roles so that gameplay can be more circumstancial.
Another one is that they have already announced about 10 different products. 10 products sounds stupid if it is just an introductory kit to 4e.
After reading this, my take is that while you could be able to use monsters from 4e, some rules will be different in the sense of being more streamlined and simple in terms of tracking effects and the like. So that if you run with "essentials only" rules the game will run a bit smoother and perhaps with less specialization for the player characters so that they will do more things.