Please let us know what you mean by "space maintainers." Do you mean an orthodontist or dentist has recommended that she have spacers put between teeth? That would be how I might interpret it but the post needs more details. Thanks.
If this is about something dental -- I would absolutely get a second opinion. Most orthodontists will do one initial free evaluation for you. Not sure if dentists will do one for free.
Is it possible your child's x-rays show that certain permanent teeth simply are not there, and will never erupt, so the dentist wants to do something so that those spaces are not crowded by other teeth --? Can't imagine any other situation where they'd say to do something for a child this young. They can see what permanent teeth are or aren't present, even at four, and it's not at all strange for some permanent teeth to be missing (never develop). But again -- hard to know what you mean or what you were told.