Mri showed suspected subependymoma 13 x 12 x 13 mm at obex of 4th ventricle indents dorsal medulla no edema and no hydrocephalus. What should I do?

Have it watched. Since the lesion is not causing symptoms at this time you may opt to have the lesion watched through serial mris. However there are reports of rapidly growing previously asymptomatic ependymoma's. So my advice would be to see a neurosurgeon to discuss your management.
Brain tumor. What did your doctor suggest? A brain tumor is not a suitable topic for casual consultation. See your doctor asap.