Thank you for your query,
It seems the same bug caused diarrhea in your 4 months old child.
Continue to give Pedialyte
to replace the losses of water in stools and to maintain hydration.
Probiotics like lactobacillus
or Saccharomyces can be give orally after the doctor advise.
Monitor his hydration by activity, urine output, and tongue moisture.
Stool after each feed is due to exaggerated gastrocolic reflex which is a normal phenomenon in this age group.
Consult your Pediatrician
with stool culture report or if loose stools worsen or child has signs of dehydration
Hope I have answered your query.