Want to switch the SIM card from my iPhone 4 to 6s plus
If you used iCloud in your iPhone 4, it's most likely you will still have your contacts and photos in your 6S Plus if you'll use the same iCloud account. If purchased music from iTunes and apps from the App Store, you will still be able to have them on your new phone. However, chat message history depends on the app developer. Some have servers to save your messages in and make them available in case the app is installed in another phone/platform (like Skype), and some just save your messages locally (on your phone) which makes them unavailable to other installations of the app.