Ideally you'd just edit them on a computer in Premiere or Final Cut or whatever you like, and either use a 4:3 timeline or 4:3 export settings. If it's full 1080 HD the 4:3 timeline/export size is 1440x1080, and if it's 720 HD the size is 960x720.
In InShot, you can import the clips, then press the Canvas icon, and make it 4:3 in the Ratio tab. Then switch over to the zoom tab and slide it up (I think it'll be 50 here no matter what). Then just export there. You'll probably lose a little quality this way, but it wouldn't be too noticeable especially if it's for Instagram or something.