Maybe:
dd = cell(1, 4);
for i = 1:4
url = data{i, 1};
dd{i} = listAllDirectories(url, folderCellArray, saveFileName)
end
But what happens inside listAllDirectories? The name "saveFileName" might mean, that something is overwritten there. Then either instruct this function to append the data. Or use a new file name in each itereation:
for i = 1:4
url = data{i, 1};
saveFileName = sprintf('try%03d.xls', i);
dd = listAllDirectories(url, folderCellArray, saveFileName)
end