Because cineál is followed by the genitive case and the genitive form of milis with a masculine noun (cáca) is mhilis.
I'm not sure why you think that teanglann.ie says "if it was "cineál mhadra" it would be lenited".
Cineál causes the following noun to be in the genitive. Both cáca and madra are, as you point out, 4th declension masculine nouns, and their genitive forms are also cáca and madra - they are lenited after an in the genitive - blas an cháca, saol an mhadra bháin
Yes, I was not very clear.
What I was wondering was why is caca not lenited in " cineál caca".
I must have seen "cineál mhadra" written somewhere I am not too sure where now. And thought it was equivalent.
Thanks for the explanation, I'll bear it in mind when it comes up again...
I didn't say that there are no sweet cakes. I said that English doesn't require the adjective "sweet" when describing a food that is served as a cake. There is no need to translate the individual word milis when translating cáca milis into English, because when you go into a "cake shop" to buy a "cake", you're buying a cáca milis.