It is also the fifth purchase made since Four secured a £10m investment from the Business Growth Fund last year, and the second following a further £5m from HSBC in April 2016.
Rain was formed a decade ago and generated £1.6m in fees in 2015, with retail clients including the New West End Company and Centre MK, and luxury clients such as Walpole British Luxury and London Luxury Quarter. It has worked on high profile campaigns such as the launch of new stores for John Lewis and more than a dozen Christmas launches on London's Oxford Street.
Rain will rebrand as Four Rain, with founder Delia Hyde becoming CEO and joining the Four operating board. The company said it expected Four Rain to work closely with many of its new sister practices.
Four CEO Nan Williams said: "The expertise and experience of the team at Rain strongly complements the existing offering at Four. There is an excellent cultural fit, great potential for cross-working and the immediate opportunity to enhance our offering in the retail, destination and luxury sectors. We’ve long been admirers of Rain’s work and we’re looking forward to welcoming it into Four."
Hyde said: "Becoming part of Four Communications Group as its retail, destination and luxury specialist provides us with both a bigger stage to operate on and the opportunity to work with other Four services including digital, design and media planning and buying."
Four has been expanding aggressively over the past five years, acquiring bgb in 2011, Colman Getty in 2012, Kinross + Render in 2013 and Consolidated PR in 2014. Last year, Four acquired MSA Media, Broadgate Mainland and Welsh integrated agency FBA.