how can i copy .mp4 files from iphone/ipad to pc?

You can copy purchases from Apple to your computer's iTunes via File > Transfer Purchases. For music bought from other sources and/or copied from CDs, then if they aren't already on your computer and you don't have them on a backup, then you will need to find a third-party program for your computer that will copy them off the devices (iTunes won't copy them)

No, I haven't used any so I'm not aware of the names of any - a search on google might find something.

Importing Personal Photos and videos from your iOS device to your computer.

http://support.apple.com/kb/HT4083

FYI

Copy Photos (& Videos) Between iOS Devices

http://tinyurl.com/cnz95bc

 Cheers, Tom 😉