If there is no problems such as a uterine infection or that she had some other type of trouble at calving time. Cows should come into heat around 45 days. Some cows come into what some people call a "silent heat" at around 19 days. Usually this is not detectable and they don't settle, but I have seen in happen. So the cow with a 4 month old calf should have been in heat about 75 days ago, if she's in good condition. We want calves on the ground, at the earliest, March. So we keep the bull out of the herd for almost 3 months after calving so we don't have calves in snowdrifts.