More importantly a rash during pregnancy may be related to the current pregnancy of 4 months. The morphological pattern of the rash is helpful & thus an opinion - evaluation by a Dermatologist is most helpful along with discussion with your Obstetrician. Correlation of the rash w/ clinical findings during a pregnancy exam is essential. A current ultrasound of the baby is necessary & should be correlated w/ frequent follow-up evals in regards to fetal viability should be done too. If a problem is of concern then consultation w/ a high - risk OB specialist should be done & sometimes transfer of a patient to a tertiary care medical center may be needed too.
Google "dermatoses of pregnancy" for more info of initial help BUT CONFIRM W/ MEDICAL SPECIALISTS. Hope this helps.