If it is away from its mother feed with puppy replacement milk in a small bottle it is available at many pet stores. Over the next few weeks introduce her/him to solid food and water. But besides that keep him/her warm dry active happy and you many want to take it to the vet for a check up
A 4 week old dog of any kind should still be with its mother. Unless this puppy is for some reason not with its mother it should be offered moistened dog food about every 3 or 4 hours and always have water available. Usually puppies start nibbling moistened dog food about 4 weeks. The dry puppy food should be very wet but not soupy. And of course the puppy should be held/cuddled/loved quite often especially if it is not with its mother. The mother usually is still caring for the puppy at this time. Start gently grooming your puppy so it gets used to being combed/brushed. Later when it needs to be groomed it won't be accustomed to it and may not like it very well and Shih Tzu's need to be groomed when they get a little older.