At 4 months of pregnancy, a dry cough for a week is usually due to a mild infection or irritation, and most cases improve with simple care; menthol-based chest rubs can be used in small amounts externally for relief, but avoid applying near the nose and use them sparingly. Safe options during pregnancy generally include steam inhalation, warm fluids, saltwater gargles, and honey (if you are not diabetic), while medicines like paracetamol for discomfort and certain doctor-prescribed cough syrups or antihistamines may be used if needed; however, you should avoid self-medicating and always check with your doctor before taking any medicine to ensure it is safe for you and the baby. If the cough persists beyond 1-2 weeks, worsens, or is associated with fever, breathlessness, or chest pain, it is important to seek medical advice for proper evaluation.