how to shrink MP4 to smaller?

I have four 35 minute video programs that 'were' MKV and I changed them over to mp4 ( audio codec issues) and each of these 4 videos are in size 1.6 GB total of over 6GB ..
I have some other MP4 videos that are over 180 min in length and they are only about 3 GB in size..
My dvd-r is 4.7 in size to use..

so is there away of shrinking these mp4's down to a smaller size??

thankyou

MP4 files are already compressed. To reduce their size you would need to either reduce the quality or size of the video.

https://www.wikihow.com/Reduce-Video-Size

Thanks will look into this.