There's no need for the first loop since randi can create a vector of values.
r = randi([1000,9999],1,4)
Since you're using a release that includes string I'd use that.
Now you can join the strings in S together.
code = "Your code is - " + join(S, " ")
There are other ways you can "do randi with letters" particularly for your first example where you wanted something like "X7S7-6S6A-Y555-U75T"
allowedChars = ['A':'Z' '0':'9'];
whichChars = randi(numel(allowedChars), [4 4]);
selectedChars = allowedChars(whichChars)
Now turn each of those vectors into strings and join them as before.
code = "Your code is - " + join(string(selectedChars), " : ")