Partners Group becomes the project's largest shareholder, with Shell holding 20%, down from 40%, Mitsubishi, through its Diamond Generating Europe subsidiary, now owns 15%, down from 30%, while Eneco and Van Oord hold 10% each. Eneco previously had a 20% stake.
Construction of the project is expected to start by the end of this year, according to Partners Group.
"With the Dutch government committed to achieving 16% of its energy production from sustainable sources by 2023 as part of a National Renewable Energy Action Plan, we believe the project is both timely and critical in helping the country achieve that aim," said Partners Group's David Daum, senior vice president of private infrastructure in Europe.
Shell's new energies executive vice president Mark Gainsborough added: "We have consistently called for offshore wind energy at scale in north-west Europe, and want to be an active player in this. Bringing in Partners Group allows us to free up capital to invest in new projects in future."
In June, it was reported the Borssele development consortium was looking for more investors in the project.
At the time, a spokesman for the consortium said the move was "part of a planned assessment... on how to best fund the project".
The group won the licence for the site, located 22km off the Dutch coast, following a tender in December, with a bid of €54.50/MWh, excluding transmission costs.
In March, MHI Vestas was announced as the preferred supplier for the project. It is set to deliver 77 V164 9.5MW turbines to the site.
Shell and Eneco Group will each purchase half of the renewable electricity production of Borssele III/IV once it is operational.
Partners Group said it has previously invested in the 400MW Merkur offshore wind project, under construction in Germany's North Sea.
It also holds stakes in two onshore projects in Australia, the 240MW Ararat and 270MW Sapphire sites.