I want to create a project to control 4 channel relay switches from an App on Iphone via bluetooth. i bought the HC-08 bluetooth module only to find out that this only works with Android devices. Does a module exist that works with both Android and IOS or even just IOS?
You need to first learn Arduino programming and then need to interface this bluetooth module with any of Arduino boards i.e. Arduino UNO, Pro Mini etc.
Once interfaced then you can install any Bluetooth App on your mobile and can receive the data from Arduino. You can use BlueTerm in Android, search in Play Store.
The OP is asking about IOS, not Android.
The ESP32 has a core that can be programmed using the Arduino IDE and it natively supports BLE (Bluetooth Low Energy) that can be used by both IOS and compatible Android devices.
iOS only connects to Bluetooth 4.0 Low Energy (BLE).
With iOS devices I have used Bluetooth module HM-10 with success.
It does work with iOS7.0 or newer.
And it will also work with Android 4.3 or newer.
For testing you can use an app like "Bluetooth Serial" on your iOS device.