My4 week old has a rash on his face, neck, scalp and back. Also his temp is ninty-nine point one. Should we go to the er? Hes not really acting sick.

Wait day or 2. Most mild rashes with minimal fever settle down rapidly in day or 2. Seek medical advice if fever reaches 101 or rash gets worse.
Depends. Most 4wk kids have a variety of rashes due to hormones passed in pregnancy or minor skin irritants. If on saclp& back it may be an oily skin. A rectal temp of 99.1 is normal but 100.4 or higher might be germs passed at birth & a reason to visit the er even if baby is acting nl. Monitor the temp & discuss the rash with babied dr during regular hours.
Call pediatrician. Many baby rashes are not serious, some are deadly. Your pediatrician can help you decide which this is and whether emergency care is indicated.