My son is 3 years and 9 months old but his focus is only for a short time. he loves to run and always roams about. do you think it's only a part of growing up?

Probably fine: I think of attention span in early childhood as equal to 1 minute per year of age up to about 5, then it varies with the content of the material being presented. There is a reason most societies do not do sit down school until the age 5-6yr.

Most likely: Children at that age are quite energetic and run around a lot to spend that energy especially boys. Give him ample space at home or take him outside and let him play as long as he is not destructive or disruptive. Attention and focus is quite short at this age. A rule of thumb is they can focus for as many minutes as old in years they are.

Normal for age: Boys of this age are curious and active. It is part of their normal development. Speak to your pediatrician as to what to expect over the next 6 to 12 month developmentally.