You must provide the Base SDK as iOS 5.0, the deployment target as the version of the OS you want to run (4.2?).
Most importantly, you cannot use the APIs / API libraries introduced in iOS 5.0 . If you want to use them, make sure that they are called only if the operating system answers them. If you call any methods that are not in iOS 4.2, you should first check to see if they are available for the OS to work. This question may help you there. This post may also help.