What video format does iPhone 4 output?

I have to send files to a friend and he prefers mp4

Ok first, that expression is incorrect. Iphone does not "output" a video format. It plays a video.

1) as a direct answer to your questoin, the best video format for iphone is mp4 or h264.

flv, mkv are incompatible with iOS. I always have to conver flv videos to mp4.

which is why i use the ipad videora converter.

anyways your friend seems to know iOS well. Since he "prefers" mp4.

best thing to do is convert the video to mp4 format, and then give it to the friend via USB.

email doesn't work properly with videos.

Yes, mp4 is totally fine. I always transfer MP4 videos to my iPhone. And if it is not MP4, I use Any Video Converter to convert it to the format I need. You may have a try. It should works.

Videos taken on iPhone use mp4