When to size up bottle teat

When did you size up your baby’s bottle teat? My baby is coming to 4 months, using phillips avent. (Not breastfeeding) Still using teat size 1 (0month+). Do i need to size up or can just continue using smaller size teat if baby have no issue and does not fuss when drinking? My baby has reflux too, and i read that slower flow might help. #FTM