Hi V.,
I am a retired OT (occupational therapist). Check out the following Website to see what developmental milestones are appropriate for your child's age.
http://www.unce.unr.edu/publications/files/cy/2006/fs0677...
Hypertonicity basically means increased muscle tone, i.e. higher or "tighter" tone than is normal, as opposed to hypotonicity, which is lower or "floppier" tone than is normal. Keep in mind that there are ranges of normal--yours can have higher or lower tone than another baby and still be considered in the normal range. Extremes in muscle tone can interfere with your child's ability to move and achieve these physical milestones. In my experience, I saw this most often with children who were diagnosed with Cerebral Palsy. However, if the pediatrician referred your 4 month old for PT, then it's important that you ask him/her what it is about your child that warranted the referral. I must say, though, that I'm impressed that your child's doctor noticed this at a young age and was on the ball to refer you to a PT. I would suggest that your child receive OT as well, because the combination of therapies and therapists working together can maximally benefit your child. Keep in mind that all public schools have special education departments that serve infants with developmental delays (for free). If I were you, I would call the Special Ed. Dept. in your school district and inquire about their Infant program and have your child evaluated to receive these free services from their qualified and licensed therapists. To be honest, the children who receive the most therapy from a very young age have the best chance of achieving success (that's why both school and private therapy are a good combination, and you'll need certain info in order for insurance to pay). All the basic sensory-motor skills are the foundation for higher learning, such as academics, which is why the schools have these programs, and they are federally mandated. Once your child is in these school-based programs, it becomes very easy to network with other parents, which is your BEST BET for gathering pertinent information. You can buy these 2 books from Amazon.com:
Sensory Secrets: How to Jump-Start Learning in Children, by Catherine Schneider
Sensory Integration and the Child, by A. Jean Ayres
Take a proactive approach--find out all the info you can about your child and take life one step, one day at a time. There are fun things you can do at home to help your child, and the therapists will give you suggestions. Hypertonicity is a very general diagnosis--a "catch-all" phrase, if you will. You may need a more specific diagnosis to get your child the services he/she may need and deserve. Good luck! jenifer