Can a pregnancy test be wrong at 4 months?

A prgenancy test at 4 months should be positive if you are in fact pregnant. The pregnancy test is based off BhCG levels which are high at 4 months and should result in a positive test.

The real question is why are you taking a pregnancy test at 4 months? Were you pregnant and now you think you miscarried? Routine prenatal care should be well underway at this point and a pregnancy well established. If you had a positive test and now a repeat is negative it is likely a miscariage happened some time ago.

I would advise seeing an OB doctor and providing a full history and copies of labs and ultrasounds if availble for full evaluation.