The Australians set off quicker as usual but were reeled in over the final 300 metres by the defending Olympic and world champions, who won by a half-length in a time of 5 minutes, 58.26 seconds. The Netherlands were a distant third but qualified.
The men's four rivalry between Britain and Australia is one of the biggest in rowing so the final, which should be a straight shootout between the two flagship boats, will be a fitting way to end the eight-day regatta.
The United States won the second semifinal Thursday in 6:01.72, ahead of Greece and Germany, and could yet be a factor in the final.
Perhaps boosted by the historic gold medal won by the women's pair of Helen Glover and Heather Stanning on Wednesday, Britain also impressed in the men's and women's lightweight double sculls in changing weather and slight crosswinds in Windsor.
Katherine Copeland and Sophie Hosking were the quickest qualifiers by more than three seconds from the women's semifinals.
And with defending champions Zac Purchase and Mark Hunter winning the second men's semifinal, it meant all 13 of the crews Britain entered for the regatta qualified for the finals — a demonstration of the host nation's strength in depth.
Denmark's Mads Rasmussen and Rasmus Quist were the fastest through to the men's final, however, after winning the first semi in 6:33.25 — nearly 3 1/2 seconds quicker than Britain.
Belarus great Ekaterina Karsten kept alive her hopes of winning a medal at a sixth straight games by qualifying for the final of the women's single sculls.
The 40-year-old Karsten placed third in her semifinal, won convincingly by world champion Miroslava Knapkova of the Czech Republic in 7:42.57, to progress. Kim Crow of Australia, who is also competing in the double sculls, was second.
There were three finals later Thursday — the men's double sculls, lightweight men's four and the women's eight featuring the U.S. and Canada.