Firstly love your daughter’s name and hugest congratulations on your baby boy 
Now as a parent I did immediately think of the TV show I’m also not overly keen on repeated initials so if it was me I wouldn’t go with Max. Still that’s just my thoughts ultimately Max is a great name and has that laid back classic feel of your older children’s names.
Looking at your other options Finn jumped out at me as a great option. Finn is handsome, soft and sounds sweet with R.uby & M1les. I also like Jack and Arlo they are sweet timeless choices of a similar vibe to the kiddos.
You’ve had lots of lovely suggestions so count any repeats as me being in agreement with the berries
R.uby, M1les and…
Cormac ~ I feel like this name makes a great alternative to Max.
Toby/Tobias
Elliott
Samson
Abel
Jonah
Silas
Callum
Elijah
Isaac
Otto
Oscar
Jasper
Henry
Arthur
Tristan
Jude
I also think using a longer version of Max would take away that associate a little and Maxwell does fit nicely.
Hope this helps