My 4 week old grunts, strains really hard and turns red the whole 2 hours between feedings. What could this mean?

Varied possibilities. Grunting and straining after feeding particularly with pulling up of the legs may represent an immaturity of the autonomic nervous system resulting in uncoordinated intestinal contraction and relaxation resulting in a crampy pain. This might be considered "colic". If it is associated with arching of the back and head turning this might be esophageal reflux.
Normal vocalizations. Infants make a variety of grunting & squeaking sounds on occasions with no specific agenda.They have no social conscience so they don't care if the sounds bother others.They need the assistance of their breathing muscle to help push poo along & often run a burp thru their pursed vocal cords. Enjoy the music & don't put adult reasons behind baby sounds.