4 months is still very very young. If you do a little bit at a time and reward them eventually it will get better. If you want to brush him and he won't, even if he lets you do two strokes with the brush reward reward reward and praise. My AprilLove was exactly this way when she was a pup. Now she has long hair and lets me brush her, doesn't jerk her head all around (lol, I'm having memories of what it used to be like), sits patiently, knows when to lay her head down to do the topknot/ponytail, lets me wash her face, etc. I guess I am just a very strong advocate for patience and repetition, all of a sudden, one day everything is going the way you hoped and you have a happy little furbutt in the process!!
Have never used force or pinned them down, just kept at it every day until they realized, hey this isn't so bad