Dumb question time. I bought an iPhone 4 on eBay and thus far am loving it. I'm due for an upgrade in August and have plans to upgrade to the 5s/6 at that time. My question is will the apps I've already purchased transfer to my new iPhone?
Yes. Any app you've ever purchased is yours, so even if you delete it from your device, you can redownload it for no charge.
Turn iCloud backups on Settings, iCloud, Storage & Backup, and then change the toggle to On for iCloud backup.
Then when you get a new device, you'll have an option to restore from an iCloud backup, you enter your iCloud login and password and it puts everything back with the exception of password (email accounts, wifi passwords, etc). The accounts themselves will be restored, just not the passwords.