No problem weaning at 4 months. I have done it many times for a show heifer.
Generally, I will pull the calf into the barn & keep them separated during the day, offering the calf some grain, then let her suck during the night. I do that for a few days, then complete wean. Not saying you NEED to do this, it just gives the calf a chance to start on grain. but, if you have been graining the dam, has the calf been eating with her?