I am going to assume this is a serious question????? The little male's will have a tiny penis that is totally covered with skin but will be attached to the stomach (whereas a human male will be free hanging). The males little "part" will extend about 2 or 3 inches up the middle of the stomach beginning between the hind legs, there will be a small ridge the entire 2 or 3 inches with a tiny loose nub at the end where the urine will come from.
how can you tell which is a boy and which one is a girl they are a week old, also is the last born always the smallest puppy. because she had 4 puppies 3 are the same size and one is the smallest. could the smallest one be more chihahua than terrier
The female will have a little loose flap directly between her hind legs. There will not be a ridge up her stomach.
The fact that the last puppy is the smallest has ABSOLUTELY no meaning except the order of the birth.
The puppies will all have the EXACT amount of genes 1/2 Terrier and 1/2 Chihuahua.
Example: A mother has 4 children, she is dark skin Spanish her husband is very light skin Swedish. The babies will all be genetically the same ~~ 2 have blond hair and are very tall, 2 have black hair and are rather short, they are still the exact same, some just "express" the height and color of the opposite parent.
Take care of the babies, be sure they are warm (as in the house), and that the mom has extra food and clean water now that she is feeding 4 babies.