i think you need to look into this more
majority of apps will pause and resume upon opening new one in OS4 . there is no support for background multitasking in apps like twitter or IM
Its all based around push notifications and local notifications

from engadget
11:22AM We just asked Steve and co. about the lack of anything that can keep track of a timeline for something like a Twitter or IM app. Scott says that the new save states for applications will be much better than the current situation. Steve also said that they stand behind push notifications and still think it's the right choice for this platform. Then we asked about widgets or glanceable information and why it hasn't been implemented on this platform or the iPad. "Apple sort of pioneered the idea of glanceable information like widgets and it seems to me you've gone to great lengths to avoid something like that in this new OS. Can you tell me a little bit about why you've decided that? With something like the iPad, it seems perfect opportunity where you have a lot of real estate." Steve: "We shipped on Saturday, and rested on Sunday." (lots of laughs) We pressed and asked if it was possible. Steve: "Anything is possible."

anything not audio, VOIP or GPS will not multitask like you think like the keywords in this picture show.
http://www.blogcdn.com/www.engadget....204-rm-eng.jpg