At 4 pounds she is about 1.81kgs, so she ingested about 55.24mgs per kilogram of body weight (1/2 of a 200mg tablet = 100mgs, 100mgs/1.81kg = 55.24mg/kg) which is a toxic amount.
From the ASPCA database:
◾25-125mg per kg of body weight – symptoms include vomiting, diarrhea, abdominal pain, loss of appetite, stomach ulcers;
◾more than 175mg per kg of body weight – the above symptoms plus blood in the stools, acute kidney failure;
◾more than 400mg per kg of body weight – the above symptoms plus seizures, coma, disorientation;
◾more than 600mg per kg of body weight – death.
Given the amount she ate I would expect to see vomiting and diarrhea, loss of appetite, and possibly bloody or tarry stools from gastrointestinal ulcers and bleeding. These symptoms can take as long as 24-48 hours to show up, although in many cases they start to feel poorly within 12-24 hours.
My hope is that she vomited up part of the pill. If she vomited within 1-2 hours that is possible. After that period of time the stomach was likely emptied and the pill was too far in her gastrointestinal tract and likely being absorbed to get back out by vomiting.
Ideally she would see her veterinarian and get started on stomach coating drugs like sucralfate, acid reducer medication, fluids to support organ function and injectable anti-nausea drugs as well as a bland diet.
At home now to try and decrease acid production in her stomach and lessen any possible symptoms you can give either:
1) Pepcid ac (famotidine) at 1/8th to 1/4 of a 10mg tablet for a pup her size every 12 hours
This acid reducers is found over the counter at the drug store. They should be given for at least 2 weeks to help in healing.
You can feed her a bland diet for the next several days. A homemade bland diet is a mix of 1/3 boiled minced white skinless chicken or lean boiled hamburger and 2/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow and get in plenty of fluids. Feed small meals frequently.
Things to watch for that mean trouble include continued vomiting, especially blood tinged, dark, black or tarry stools, or frank blood in the stools, lack of an appetite, increased water consumption and urination.
Please let me know if you have any further questions.