Apologies for the late reply. I work in the paediatric setting and we usually calculate doses in the following manner.
To continue with your example, once we diluted the vial of pipercillin/tazobactam 4.5g strength vial with 20mL water, we would withdraw the required dose right away – for example if the patient weighs 10kg and we are giving 100mg/kg/dose = 1gram required, we would take out 5mL then further dilute this into a compatible fluid then infuse over 30 minutes. In which case, this 5mL volume is very small hence negligible. This is usually the practice for most drugs, unless huge volumes are required to be diluted further which is pretty uncommon.