That’s twice as many as have seen Take That’s recent Progress Live Tour and nearly half a million more than go to the cinema each week.
The support members give, helps look after more than 300 historic houses, 250,000 hectares of land and 700 miles of coastline.
Members enjoy being able to discover these amazing places and a total of more than 90 million people  visit the National Trust each year – 17 million to houses and gardens and around 75 million to coast and countryside.
Fiona Reynolds, Director General at the National Trust, said: “Four million members is a powerful indication that our mission to look after special places for ever, for everyone remains as relevant today as it was in 1895.”
Fiona Reynolds continued: “People join us for many reasons – a love of their local place, a passion for beauty, for heritage and fine art, for surfing and cycling, or simply the joy of family time together. What all our members have in common, though, is a love of beautiful and historic places.
“It’s encouraging to see that people’s love of places and the simple pleasures they bring remains as strong, if not stronger, in hard economic times as it does in the boom years”.
In 1895, when the Trust was founded, there were 100 members, paying an annual subscription of 10s; life membership cost £20, and honorary membership was given to those who donated £100 or more.
Membership in the first few years grew steadily with 260 members in 1900, 450 in 1905, 630 in 1910 and 670 at the outbreak of the First World War.
Since then membership grew steadily to reach one million in 1981, two million in 1990 and three million in 2002.
Last financial year membership provided the charity with £120m which was ploughed back into thousands of conservation projects throughout England, Wales and Northern Ireland.
Fiona Reynolds added: “We owe a massive thank you to our members. Their support helps fund the vital conservation work that keeps the special places in our care open for everyone to enjoy.
“In England you are never more than 40 minutes away from somewhere looked after by the National Trust.”
“From the smallest sculleries and garden grottos to towering castle turrets and the wild expanses of Lake District countryside, the National Trust enables everyone to enjoy these beautiful, intriguing, and exciting places for ever.”
To celebrate the milestone the National Trust is inviting visitors on the weekend of 15 and 16 October to share their experiences in order to create a virtual time capsule of the occasion.
Visitors can share their comments and photos through the National Trust’s Facebook group at facebook.com/nationaltrust and Twitter feed at twitter.com/nationaltrust.