I found this great little app "LosslessCut". Allows you to merge, concatenate, full or just selected portions of various video files (including MP4, of course). And of course as its name implies, does it without recompression! Very fast! Great for quickly joining video segments from Action 3 if you don't want to mess with DaVinci or some other big name editing suites.

Available for Mac and Windows both via app store downloads or as a package. The app store downloads are paid versions but the packages are not (DMG for Mac, Zip for Windows). Read the description and scroll down for the various downloads.

Enjoy!

https://github.com/mifi/lossless-cut#readme


"LosslessCut aims to be the ultimate cross platform FFmpeg GUI for extremely fast and lossless operations on video, audio, subtitle and other related media files. The main feature is lossless trimming and cutting of video and audio files, which is great for saving space by rough-cutting your large video files taken from a video camera, GoPro, drone, etc. It lets you quickly extract the good parts from your videos and discard many gigabytes of data without doing a slow re-encode and thereby losing quality. Or you can add a music or subtitle track to your video without needing to encode. Everything is extremely fast because it does an almost direct data copy, fueled by the awesome FFmpeg which does all the grunt work."

A great tip