For a 4 year old Milk of Magnesia dose is:
Oral Liquid: 5 to 15 ml orally once a day or in divided doses.
Chewable tablet: 2 tablets orally once a day or in divided doses.

Milk of Magnesia (Magnesium hydroxide) can be prescribed or recommended by a doctor as a laxative for occasional constipation in children.

Age Group Pediatric Doses for Constipation are given below.:

Oral liquid:
1 year or less: 0.5 mL/kg per dose.
2 to 5 years: 5 to 15 mL orally once a day or in divided doses.
6 to 12 years: 15 to 30 mL orally once a day or in divided doses.
13 to 18 years: 30 to 60 mL orally once a day or in divided doses.

Chewable tablet:
3 to 5 years: 2 tablets orally once a day or in divided doses.
6 to 11 years: 4 tablets orally once a day or in divided doses.
12 to 18 years: 8 tablets orally once a day or in divided doses.

For more information see:
https://www.drugs.com/mtm/magnesium-hydroxide.html