What can I do my 4 months old never poops only with glycerin ?

Make sure you are drinking plenty of water and give him extra bottled water. The 2 main causes of constipation are dehydration, and sugar. So also look at your diet and I am assuming baby is not on solids yet. Also, is he uncomfortable? Some babies don't poo for a few days and are fine, but obviously if he is uncomfortable he needs help. The trouble is the more you give him glycerin, the less likely his body will learn to do the work itself. So it looks like you need water, water, water.....