Compatibility with Version 4 of iPhone OS for cloud computing

I am now using iOS 5 and naturally, have integrated my directory and calendar application to Apple’s cloud computing system. I am now thinking of re-installing the fourth version of iPhone but retain the cloud computing.
If I pursue this, will my applications still successfully integrate using Apple’s multi-media player or should it really just be within that cloud computing domain?
I would like to know the outcome so that I can decide. I hope you can help me.
Thank you so much.
