3-4 month period

Overview of development at 3-4 months

PDF download    Notes on infant perceptual-motor development 3-4 month period
Head control and pull-to-sit    Pull-to-sit birth to 4 months    
Supine development    Supine 3-4 months 
Sitting   4 months   
Prone   4 months   3-4 months videos
Hand function  Foundation manual behaviors birth-5 months    Will at 4 months   

Assessment and intervention strategies and training 
Supine head control: assessment and training
Sitting   Assessment overview 
Prone    Assessment and intervention strategies 
Hand function  Observation and assessment guidelines   Setting up a play gym to encourage hand use

More resources 

DR Glen Trip        Your baby at 4 months