On a night last fall, a 4-year-old stayed up 45 minutes past his bedtime -- and it almost changed his life forever.

The next morning, the boy was scheduled to take the independent-schools standardized admissions test, a full-bore IQ test known colloquially as the ERB. But his parents, Cynthia and David, had gone out the previous night and forgotten to tell their baby-sitter to have their son in bed by 7:30. He went to sleep at 8:15.

They may have forgotten because they had little reason to be concerned. They already had a daughter at one of the city's top girls' schools. And both parents had attended Manhattan private schools themselves. They thought the knew the ropes. Besides, they believed their son Andrew* was a very smart little boy.

"I thought I'd have a choice of schools," Cynthia says. "He was a bright penny. He caught things quickly. You could see the wheels were spinning."

But when he arrived at the offices of the Educational Records Bureau on East 42nd Street, Andrew wasn't exhibiting his normal sparkle. "It makes a difference," his mother insists. "He's an early riser."

Cynthia began to worry as soon as his examiner led him off to the testing room. "I just had a bad feeling, and sure enough the tester comes out and says, 'I think you've got a tired little boy,' " she remembers. "My heart sank. It was just awful. I could have kicked myself. How could I blow it like that? The results proved me right. He didn't even pick up his pencil to do the mazes, and the tester noted this. His scores were very skewed. He did well on the verbal and abysmal on the mazes."

One might accuse Cynthia of overreacting. After all, how meaningful can a single test be -- especially when the test-takers are 4-year-olds, a demographic group notoriously susceptible to hunger, fatigue, and the sounds of traffic? Unfortunately, she wasn't. With fewer families, flush with their Wall Street winnings, fleeing the city for the suburbs, private schools have become more selective than ever. And even though admissions directors insist the test is only one variable they use in deciding which children to accept, those without high scores need not apply to the top tier.

Parents, aware of the exam's power, have turned their children's fourth year, a year that rightly ought to be devoted to imaginary tea parties and playing tag with friends, into a yearlong study hall filled with snap vocabulary and arithmetic quizzes -- at what cost to their children no one yet seems to know. "She could learn to cut a spiral at 4 or 6," observes a preschool head, referring to one common developmental test. "But if you lose the time for a tea party and the kind of creative thinking and social interactions, it's gone. They could learn to read at 4, 5, 6, or 7. But if they're forced to read at 4, they're missing something. There's only a certain amount of time and emotional energy.

"Every January, I have very sad feelings about the process," she continues, "that children are made so anxious, that parents are made so anxious. I'm really a teacher. My job isn't about next year. My job is to make it right today.

"I used to think things would change," she concludes. "I used to think it would get worse and worse and then get better. But now I know it gets worse and worse and worse and worse."

Some ambitious families, believing their child's destiny, not to mention the family's honor, depends on scoring in the 98th percentile, are even having their toddlers professionally tutored to ace the test. "It's no longer a level playing field," says Elisabeth Krents, the director of admissions for the Dalton School's First Program. "There are people who are teaching the test to 4-year-olds. As a school that prides itself on ethics, I have a real hard time with what's happening."

"The tests don't measure creativity," says Yale psychologist Alan Kaufman. "They don't measure social intelligence. They measure a relatively small band of mental functioning. They can't predict life success."

Krents recalls the mother -- perhaps less sophisticated than some -- who admitted that her kid had been tutored. "She proudly told me, 'The ERB coach I got told me my child only needs coaching three times a week as opposed to five times a week.' "

Krents doesn't know where the child ended up for kindergarten, but it wasn't Dalton.

Though the exam is colloquially known as the ERB, the Educational Records Bureau is simply the organization that administers it. The actual exam is the revised Wechsler Preschool and Primary Scale of Intelligence -- the wppsi for short. It is composed of verbal and performance subtests designed to explore everything from a child's vocabulary to his ability to perform fine motor skills and solve math problems. The test usually runs between 45 minutes and one hour in length.

"The ERB test has been described as shooting flies with a bazooka," says John Dexter, the head of the Trevor Day School. "It's a full-battery IQ test. It's quite a good test. Whether or not it's a first-class admissions tool is often discussed."