This is not a question, actually. Just wanted to share what I found out in case it helps others and hear comments if any.

If you are like me, you will sometimes backup all your files including some MR images of disks and partitions, in order to keep an extra copy of everything.

If you use MR to backup your files, then you will notice that MR images (that is, .mrimg files) over 4GB cannot be explored as such when they are stored in a MR File and Folder backup container (that is, in a .mrbak file), because of technical reasons. They need to be restored (extracted) before use.

Therefore, this is what I do when I create a MR image: I manually set 3.99GB as Size in
Advanced settings - File size - Enter a fixed file size for the image.

I've just tested it and it works. Now I can explore my large MR images stored in a MR File & Folder backup without having to restore them. 

Please note: 4GB image splitting is automatic in case you save to a FAT32 volume, but I usually save to NTFS, so I need to set Size manually.

Please also note: "Incremental retention rules will not be run if backup files are split. This can be caused by setting a fixed file size or if the destination file system is FAT32" (that's what I read in MR).