r=4, n=21, 3358 codewords.
Construction using the 4-identifying code with n=19 and 835 codewords, which has
X2={16380 47774 227344 234571 252342 294552 342026 347577 374269 376983 416872 422150 424572 501080 519727} (15 elements)
Y2={277212 465994 227606 505820 377241 228879} (6 elements):
3358 = 835 x 4 + 6 x 3.