You need a SIM card to activate it after you erase it. You shouldnt need a SIM card when you are connected with wifi with an iPhone that is already activated. The SIM card doesn't need to be activated or have service. Just put any dead sim you have in it. And then do the erase all content and settings when connected to wifi.
If this does not help. And you believe you need an active sim, one other possibility is to go to iCloud.com and erase it from the website, and then tell the website to remove it from iCloud.
That way , when the iPhone 4 does connect , it will automatically erase itself
I often get worried selling iPads on ebay. I sold my my ipad locked to iCloud because I was so worried about Someone in the Mail stealing it. I made the guy email me saying he signed it, before I released it.
I wouldn't bother deleting your movies manually. The iPhone should let you erase all content and settings , and that way it will clear all your stuff and be like a new phone to the next owner