As part of "an exciting modernisation and investment programme taking place across the Post Office network", the new branch opened its doors last Thursday at the Co-op store on Winchester Road.
"The new branch offers customers a wide range of Post Office services without altering the existing retail store environment and products available," a spokesman explained.
"The new Post Office service is located at one of the existing retail counters where customers are able to purchase their food while posting letters or collecting online shopping items. Customers will also be able to take advantage of a wide range of banking services, including cash withdrawals and balance enquiries for customers of all the main UK banks.
"The new Post Office services are available seven days a week throughout the store’s opening hours: Monday toSunday from 6am-11pm.
"This offers customers an impressive 119 hours of Post Office service a week, making it very convenient for customers to visit."
Bev Wyatt, Southern Co-op’s Post Office relationship manager, said: "As a regional, independent co-operative, we are pleased to be able to provide a much sought after service in the local community. We had large numbers of customers getting in touch asking for our help and we wanted to give something back.
"So we are extremely pleased to be able to offer this service in store and support our local residents."
It was officially opened by long time Four Marks resident and former parish councillor Derek Seaton.
Having been calling for better Post Office provision locally, he told the Herald that "years of campaigning has at last paid off and common sense appears to have prevailed."