It said the scheme for some store managers is part of a "case for change" which also includes flexible working and shorter shifts.
A spokesman for the supermarket said they were still the "feedback" from staff involved in the scheme had been "positive".
He told The Sun Online: "Since September 2023, we have been trialling a variety of flexible working patterns for managers in 20 stores, including a 4-day working week for the same pay and benefits.
"More flexible working patterns have become commonplace in retail leadership in recent years, and we are keen to test and learn different ways of working that benefit our colleagues and business."
It comes after supermarket rival Sainsbury's introduced flexible working including some four-day weeks last year.
It comes after a recent survey found Aldi beat rival Lidl to be named the cheapest supermarket of 2023, according to consumer group Which?
Aldi was the cheapest supermarket for 11 months of the year, with Lidl beating the rival discounter for one month in October, the consumer group said.
December’s results show a basket of 43 groceries was £74.83 at Aldi, narrowly cheaper than at Lidl where it cost £76.74.
Asda was the cheapest supermarket for a larger trolley for 11 months of the year, with Morrisons being the cheapest in July.
Waitrose was the most expensive supermarket for 11 months of the year for a larger trolley, while Sainsbury’s emerged as the most expensive for customers shopping without a loyalty card in September.