Apple – iPhone OS4 update – for Exchange Apple will release a new update to iOS 4 at some point that will resolve this.  Please apply the following workaround until Apple releases an update.

Apple has published an Apple KB article (https://support.apple.com/kb/TS3398) with a link to a configuration file that can be downloaded to the iOS 4 devices via e-mail or with the Safari

https://www.hakanuzuner.com/apple-iphone-os4-update-for-exchange/