There are plenty of tools I guess. I’m on Linux and would use e.g. ffmpeg. No idea what one would use on Windows/Mac, but video transcoding tools often have that feature. What you do is just basically:
make Blender create a 100 FPS video
use tool to change the video file so it says it is actually 25 FPS (without changing the video frames itself at all)
video player will play it then at 25 FPS, thus it runs at a quarter of the speed Blender thought it would
You would have to disable time remapping for that. The reason why that approach was my second suggestion is, that you would have to adjust all anims (move the keyframes and such). It would all work the other way round. You would render just every 4th frame for the time range you want in normal time at 25fps and all frames for the slomo part.