Well, I had a great day in the Cambridge Arm offices yesterday getting the information on the new core straight from the horses mouth.
Trying to cut down the presentations from Arm, Freescale, NXP would do them a great injustice, but in short if you enjoyed working with the M3 core
your going to love the M4, it expands on the existing instruction set and adds some brilliant new features the ones that stand out in my mind are:
DSP functions: Yay finally! and it looks like these will be easily accessible to us C programmers using extensions to the C language which were
refered to as "Primitives" so instead of relying on the compiler to decide if the operation should use the MAC etc you get to take control and say it will be done
in this manner. Primitives in this context are new to me, perhaps I have just fallen behind a bit but its going to be fun catching up 
Enhanced Debugging with ETM and CoreSight.. Afaik the acronym is short for Embedded Trace Macrocell and it appears to have been around for a while but hardware debuggers that support this feature have been beyond my meager budget in the past hopefully we shall see some in the sub $1000 dollar arena. (Yeah, I'm known for my optimism)..
Would highly recommend checking out the presentations on NXP and Freescales websites...
I was going to finish off with a piccy of me looking like an idiot outside the Arm offices but a colleage beat me too it and posted it elsewhere on the forums 
Only I am allowed to make myself look like an idiot... revenge will be sweet.