Iphone4 is so slow and will not play videos

There are too many cookies and files on your iPhone, and the most thoroughly method is factory reset your iPhone. But make sure you have a backup first. Follow the steps below to get a backup to iCloud:
Step 1: Go to Settings app and tap your Apple ID with your name.
Step 2: Select "iCloud" > "Manage Storage" > "Backups"
Step 3: Tap Back Up Now. Stay connected to your Wi-Fi network until the process completes. And you can also let it automatically back up in the background.
Step 4: Navigate to Settings > General > Reset and then select Erase All Content and Settings.
Step 5: Restart your iPhone and use your Apple ID to activate your iPhone with your iCloud backup.
If you think the backup process is quite slow, here I found a tutorial for your reference: How Long Does it Take to Backup iPhone to iCloud

Connect your iphone to itunes and reset your phone ios. Take backup before doing so. Hope this will help.

For any network unlocking solutions.

Consult: www.guaranteedunlock.com