Need to convert MP4 to an editable format for Premiere or iMovie

I am on a MAC and I just need to replace 3 images in the MP4, group shot of employees (2 no longer work here) and 2 images of the people that are gone.

As you are on a mac try Prores 422 (not sure iMovie will support this codec).

iMovie is an Apple product, so you need to find an Apple forum for that program

Premeire Pro will easily edit MP4 as long as what is inside the file is compatible

MP4 is a wrapper, what is inside YOUR wrapper - Exactly what is INSIDE the video you are editing?
Report back with the codec details of your file, use the program below
Free program to get file information for PC/Mac http://mediaarea.net/en/MediaInfo/Download
- when you analyze your file in MediaInfo and post a screen shot in the forum, do so in TREE view
- post your information IN your message, not as an attachment that someone would have to download

Copied from Jeff Bellune
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One of the most important lessons a video editor can learn is the difference between a codec and a file format.
A codec determines how the video is stored digitally. All codecs compress video into smaller file sizes to store it.
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They have to discard some of the video information to do that. This compression starts in the camera or software that first captures the video.
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The best quality codecs are "lossless" because when their files are played back, the video is virtually indistinguishable from the original. The drawback is their file sizes are quite large. ProRes 4444 is an example. So is Cineform.
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The most efficient codecs are ones like H.264. They throw away enough video information to make relatively small file sizes, but when they are decompressed and played back, the quality of the video is so close to the original that it appears no different to the eye. That involves a lot of number-crunching. The biggest drawback to high-efficiency codecs is the massive amount of computing power required to decompress the video on the fly for playback and editing.
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Those compressed videos have to be stored in a container that computers can recognize. That's where file format comes in. Just like this sentence I'm writing could be copied and pasted into a Word document, it could also be pasted into a simple text editor. Now, Word can read a .txt file, but Notepad can't read a .doc file. They serve different purposes. Yet the actual words I used in the sentence are exactly the same in both. The .doc file and .txt file are only containers (or wrappers) for my words.
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The equivalent situation in video are the various file formats we all know: AVI, MOV, MP4, MXF, etc. They can all contain video in a variety of codecs, but they were designed for different purposes. It's not the file format that determines quality. It's the codec inside the wrapper.
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Codec tutorial https://community.adobe.com/t5/Premiere-Pro/Adobe-Premiere-Pro-Tutorials-Video-Codecs/td-p/4788943
Codec VS Format https://community.adobe.com/t5/Premiere-Pro/Video-Formats-Codecs/td-p/5378341

Some Tutorials to help you get started
https://helpx.adobe.com/premiere-pro/get-started.html
https://community.adobe.com/t5/premiere-pro/premiere-pro-tutorial/td-p/10974406?page=1
https://community.adobe.com/t5/video-lounge/tips-amp-tricks-to-become-a-premiere-pro-power-user-with-abba-shapiro/td-p/10900149?page=1
https://community.adobe.com/t5/video-lounge/video-series-real-premiere-pro-secrets-by-cut-to-the-point/td-p/10900176?page=1
https://community.adobe.com/t5/video-lounge/podcast-art-of-the-cut-coup-53-editor-walter-murch-ace/td-p/10887889?page=1
https://helpx.adobe.com/premiere-pro/how-to/premiere-pro-nesting-sequence-cc.html