How to factory reset iphone4 without a computer

You can erase the device using iCloud under these circumstances. If you do not meet these requirements, then you will need to use a computer running iTunes.

Erase your device with iCloud

If you set up Find My iPhone on your iOS device or Mac, you can use iCloud to erase and restore it.

1. From another device, go to icloud.com/find.
2. If prompted, sign in with your Apple ID.
3. Click All Devices at the top of your browser window.
4. Select the device you want to erase.
5. Click Erase [device] to erase your device and its passcode.
6. Now you can either restore from a backup or set up as new.

If your device isn't connected to a Wi-Fi or cellular network, you can't erase it with Find My iPhone.

Simple answer is you can't.

In order to remove the forgotten passcode, you must reinstall the iOS, which requires a PC running iTunes. If you visit an Apple Retail Store, they should be able to provide a computer for you to do this.

If the device has not been backed up, you do not have a chance to do that now. The reinstall will remove all content on the device.