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Instead of managing all of this from your iPhone you need to bring your computer into the equation. How you want to do this is going to be up to you, but you may just want to utilize iPhoto. Plug in your iPhone and then have iPhoto open to identify the video files that you have already recorded with the iPhone 4's 720p HD
video. From here you will want to select to import the videos from your iPhone, but also prevent it from deleting the videos from your iPhone as you may want to reference them again. This will put the original 720p HD videos onto your computer without downsizing them. From here you can then log in to your
YouTube account directly through your web browser and initialize an HD video upload. When you upload the HD video directly from your computer it will come in full size and will not be downgraded as it was from your iPhone 4. This process is going to have to be done no matter what size the video is from your iPhone as it is going to downgrade all videos, no matter what length or overall file size.