At 4 years, your child is still quite young and at a higher risk of catching side effects easily. Most doctors feel that cold syrups are not really effective, and it is best to let the cold run its course for 5 to 8 days.
If however, your child still suffers from the cold, it is best to consult a doctor. In the meanwhile, you can try a simple and sometimes very effective remedy of mixing a tsp of honey and a tsp of ginger juice in a glass of warm water and letting your child sip on the same.