Pumped breast milk

Can I keep freshly pumped breast milk in bottle warmer like avent brand for 3-4 hours till when baby wants it ? Also can I keep the milk back again in Warner if baby doesn’t finished it during the feed as long within 4 hours from when pumped? Wanted to ensure I’m not doing any mistake as baby started being fussy so wanted to ensure not because of my warming BM method ? #pleasehelp #firstbaby