mmv works well for uncomplicated file renaming.

mmv '*-*-*-*.mp4' '#1-#3-#4-#2.mp4'

It may also be helpful to have some more reference points to be sure not to change the name on something else, especially where there may be other files in the same directory.

mmv 'sluno-[0-9]*-call-[a-z]*.mp4' 'sluno-call-#3#4-#1#2.mp4'

             ^  ^        ^  ^

             1  2        3  4



sluno-20201027-call-metroua-status.mp4

      ^-------      ^-------------

      1   2         3       4

If I were to do it with the bare minimum, mv and bash I would go with a for-loop and bash builtin BRE matching.

for i in *.mp4; do 

    [[ $i =~ ^sluno-(.*)-call-(.*).mp4 ]] && 

    mv -- "$i" "sluno-call-${BASH_REMATCH[2]}-${BASH_REMATCH[1]}.mp4"; 

done