Uhmm... We have a 4 month old puppy...
She's been healthy at start but then we had 2 new puppies, one of them died...
After a few days, the little puppy ( not the 4 month old ) got sick... She's like coughing and has a cold too.
She's not as active as before and only eats little.
After a few more days, our 4 month old puppy started to be pale.
Then she had mucus on her nose too ( colds, perhaps? )...
She started to become lethargic.
She won't eat a thing... I'm worried. It's already been days. She is really skinny now...
I tried tempting her to eat but no, she wouldn't do a thing. She wouldn't even open her mouth.
Actually, my mom brought the younger puppy to the vet and gave antibiotics...
My mom also told about our other puppy who has the same symptoms as the younger one. I guess the older one is taking the same medicine as the 1 month old puppy...
They have stopped with the cold thing but the older puppy's appetite was really lost. I couldn't feed her. I couldn't chuck in solid foods in her mouth... She won't open but I've tried forcing her to drink milk...

I don't know what to do...

Have these puppies been vaccinated? It could be a number of things, one of them being distemper which is deadly. You cannot let a puppy go a week without eating. You need to get it to a vet ASAP.

This puppy needs to see the vet himself. The deceased puppies vet visit holds no significance for the surviving pup. This could be any number of things that only a vet who can examine him can tell you. Again, any vaccines so far?
What type of milk are you trying to feed him? If it is cows' milk then stop, it can cause diarrhea, not something you want in an already compromised pup.
You will be much better off to try puppy replacer milk or even some gatorade or pedialyte until you can get puppy milk.
You really need to get this puppy to the vet.

Kind of sounds like this dog needs veterinary support, not human support.

Force feeding an already sick dog solid food is not going to work. You need to slurry the food and syringe feed her.

This dog needs a vet. Young puppies can die from simple things such as kennel cough... However, I'm sure your puppy has something much worse than kennel cough.. maybe Canine Influenza which is a deadly disease in healthy adult dogs.

Either go to the vet to help her out, or have euthanized... Or I suppose you could watch her die slowly...