I started taking vyvanse (lisdexamfetamine) 70mg for 8 days now, but i haven't had a bm for 4 days. what should i take to have a bm?

Vyvanse (lisdexamfetamine) not culprit: Stimulant medications usually cause diarrhea instead of constipation. My suspicion is that the stimulant effect of vyvanse (lisdexamfetamine) has probably made you have less intake whether it is fluid or food. This is common and you might need to increase your intake. I would give it no more 72 hours before seeing a physician (1) increase fluid intake (2) add fiber to diet (3) discuss with md/do who gave you med.

Stool softener : Stimulant medications often will cause loose stool but some people do seem to get constipated. For some it is because they are eating less due to appetite suppressing effect. Even if you are not hungry you need to maintain good fiber intake as well as plenty of water. If you cut back on coffee because of stimulant that may also be the cultprit . May need daily fiber supplement to stay regular.