There are plenty of collage apps for iOS. One of the best is Diptic from Peak Systems. It allows you to grab photos from your camera roll and combine them into attractive, framed presentations for sharing.
Now there's something new called Diptic Video, which is US$1.99 from the App Store. You can still use it to create photo collages, but it adds video to the mix. The process is similar. Select some videos that are thematically related, load them into Diptic Video, arrange them how you like, and pick a presentation (there are 35 templates to choose from). You can have four windows all with video, or three or two or one.
