Q. My daughter is worried about my 4-year-old grandson because he has imaginary friends.

He has a new baby sister since September and he has no playmates close by, but he does go to preschool three times a week for about three hours.

I think his friends are just a release for his social instincts since he is alone so much and his mother has little time for him lately. What do you say?

A.:Grandmothers don't always know better, but you're close to the mark. Unless his friends are the scary, hallucinatory variety, your grandson is just doing what comes naturally.

Studies show that a third -- maybe even half -- of all children have imaginary friends when they're little.

Even though many of these children have no brothers or sisters, they probably don't invent their friends out of loneliness. Instead a child will create an imaginary friend as his true protector; his best listener; his most loyal friend; his obedient servant. The friendships that are born in his head are part of a mental game, letting him concoct his own adventures, so he will be safe without being bored, for this child is bright enough to be a good problem-solver. This is much the way a well-coordinated child climbs a gym set -- or anything else that's handy -- and does it over and over. Children like to do what they do well, at least until they perfect it.

It's also likely that these imaginary friends reflect your grandson's age much more than the circumstances.

Three-and-a-half is the classic time for the bright, creative child to invent friends, give himself a make-believe identity or pretend he lives in a fantasy land.

By 4, a child's preoccupation with his imaginary world can reach such a peak that parents find themselves making space in the car for the invisible child, even when the real child forgets to yell, "Watch out!"

This is your daughter's chance to peek into the wild and curious mind of the young. In a couple of years her son will invent a fanciful, perhaps heroic story that puts his playmates to rest permanently and without regret. It's quite amazing. Whatever has been joined by a 4-year-old can be put asunder painlessly at 6.

In the meantime both you and your daughter need to remember that while these invisible friends are simply the products of his active imagination, the new baby is a relentless reality. That's sure to cause some jealousy.

The first symptoms appear immediately and then fade in a few weeks -- if only because friends are thoughtful enough to bring presents to the older child too -- but the problem usually gets worse at about 3 months. That's when the baby starts cooing, the relatives start gooing and the firstborn feels second-best.

He may beg to be nursed or have a bottle again or drag his magic blanket around as if it were part of him or perhaps start sucking his thumb or his fingers more. Whether he shows all of these signs or not, you can bet that the minute his mom starts taking care of the baby he either whines for attention or gets into trouble. Or both.

Most of this is nothing to worry about. The thumbsucking and the blanket may look silly or babyish to you, but they serve a good purpose. Children need their comforts just like grown-ups. And as for the need to nurse, one or two feedings at the breast or the bottle usually will satisfy the curiosity of the older child.

It's the pleas for attention that are as tough to take as they are inevitable. No parent can give all the reassurance the older child needs, but it will help if your daughter gives him extra hugs and stories and gets a sitter to watch the baby several times a week while they take a little walk or go to the library so they can just be two of them again. If she gives these treats to your grandson unexpectedly and when he is being good, rather than as a bribe or a hush-me-up, he will try to be more pleasing. This will make him feel better about himself and more kindly toward his baby sister, so they can grow up as good friends, most of the time.

It's the grandparents who can do so much to ease the transition from only child to big brother.

If you live near, you have the chance to drop in, to baby-sit, to take your grandson to the zoo or to your house for an overnight, to remind him how special he is. This kind of attention also tells your daughter that she is as dear to you as ever.

Even if you live at a distance, your grandson can be bolstered by your kindnesses. He'll like out-of-the-blue phone calls and post cards and tapes you've made of children's books that he can follow while he listens as well as family stories about his mom when she was 4 or some of the short nifty tales, fables and parables in Tell Me a Story by Charlie and Martha Shedd (Doubleday; $12.95). This gentle, slightly religious book teaches grandparents how to turn a story into a conversation and tighten their bonds with a child still more.