Is it normal for a 4 yo to be very shy? he talks very much at home and to people he knows well, but is very quiet around acquaintances and strangers.

Very normal: What you've described is not at all abnormal. Always consider personality differences and a child's development stage. These observed behaviors may likely change with maturity and new experiences, but do not be surprised if your child is later seen to be more introverted. A four year old is still "undeclared." You can respect and appreciate his unique personality type and develop its strengths.