Been taking 20 mg Lexapro (escitalopram) for almost 4 weeks now. Have started to feel very tired and dizzy. Also taking Quillivant XR. Do I need to lower my lexapro (escitalopram)?

See your doctor. Too little info. to answer question. Could be from either med's side effects, drug interaction or other reasons. Did you get a baseline EKG before starting Quillivant? If not, would suggest get one to make sure no cardiac side effects. Need to know what meds and doses you've been taking when you felt very tired/dizzy. Talk to your prescribing doctor without delay for guidance.
Maybe. at 4 weeks on the same dose, i would have expected the symptoms you describe to have disappeared and not appeared. Talk with your doctor about reducing the lexapro (escitalopram) and seeing if that helps. but a lesser dose might not be effective in alleviating your symptoms. It's possible another med may be needed.
Talk to your doctor. Talk to the doctor that prescribed your medications about this.