You cannot get iOS 8 options on the iPhone 4 by installing the firmware update because this new beta operating system stops supporting this model. However, it is possible to enjoy iOS 8 features on iPhone 4 and newer models without the upgrade.
This is pretty cool and you can get similar features without risks. It will take Apple another 2-3 months to complete its update and release iOS 8 to public. Till then you can use various iOS 8 options in iOS 7 and have no problems.
You just need to jailbreak your iPhone first which is impossible for 7.1 and 7.1.1 devices yet. If you have earlier version of iOS 7 you are good to begin and change your experience.
iOS 8 Features on iOS 7
If you wish to receive interactive notifications you can install Auki and LockInfo instead. The first tweak supports Message application and it provides you the Quick Reply and even Quick Compose option with ease. With LockInfo, you can simply check your mail box and dismiss notifications. There are no programs that offer quick reply from Facebook and other iOS 7 apps at the moment but they might be released in the future.
It is simple to type faster with QuickType or PredictiveKeyboard tweak. Each app suggests the words for completing your phrases and sentences. It is neat and fun to use.
iWidget can improve your Notification Center app and iTouchSercure can autofill your passwords using Touch ID option.
If you wish to name your conversations you can use MessageRenamer7 jailbreak tweak for iOS 7 that works similar to iOS 8 feature.
Users who need to use a third-party keyboard can install AltKeyboard, SwypeSelection, Hipjot and Fleksy from Cydia store.
If you are a big fan of Siri download OkSiri to make your personal assistant work hands-free while driving.
You can also get iOS 8 features in iOS 7 by installing CC Brightness Slider app that allows adjusting your brightness on the smartphone and tablet.
There are many more options you can get if you search Cydia. This jailbreak app store has a lot options to offer and make your iPhone 4 or other model support different iOS 8 options without update.