The birds were discovered at Eileen Gay Optometrists in Broxbourne, when staff opened the shop.
One of the birds died in the incident on Thursday, while the others are now being cared for by the RSPCA, which is trying to find those responsible.
Last year, the same shop and the nearby Bourne Beauty beauticians had budgies posted through their letterboxes.
'Prosecute culprits'
Shop manager Angela Aspinall said: "We used a net to catch some of them. Three more were flying around and we found the last one behind some of our optical equipment.
"We have no idea why anyone would do this, but whoever it is should stop."
An RSPCA spokeswoman said: "Birds are easily stressed and they would have suffered greatly from being posted through a letterbox. This was a very cruel thing to do."
She said the RSPCA would want to prosecute the culprits and appealed to anyone with information to contact them.
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