With 3 out of 4 requiring help I'd say you're due for quite a few unassisted births, statistically speaking. On an average year, I will pull 1 or 2 calves for every 50 cows.
While I don't advocate neglecting your cattle, I believe there is such a thing as too much worry and intervention. They are animals, having survived on this earth for thousands of years without us wiping their butts and noses everyday. Every now and then you will have to assist/doctor on them, but this should be the exception, not the rule. I applaud your enthusiasm and the care you are willing to give your animals, they should all be so lucky. But in the same notion, I don't believe in forcing nature's hand, just be prepared to play the cards when they are dealt.
Good luck with your heifer, she should do fine on her own. We've had similar situations where everything turned out just fine.
cfpinz