The funding is in the form of a loan from HSBC. It is in addition to a £10m equity funding package from the Business Growth Fund, the growth capital fund backed by five major banks, announced last year.
Four CEO Nan Williams said the new funding package "gives us the mobility to acquire new businesses".
In 2015 the group acquired financial PR agency Broadgate Mainland; Wales-based PR, design, marketing and public affairs agency Francis Balsom Associates, and healthcare media planning and buying agency MSA Media.
Previous acquisitions have included Consolidated PR, Colman Getty, bgb and Kinross+Render.
London-headquartered Four Communications has a total of seven offices across the UK and internationally, including in Dubai and Abu Dhabi. Launched in 2001, the company now employs around 240 people and generates annual turnover above £40m.
Tom Wood, HSBC regional co-head for corporate banking in London, said: "Following this latest tranche of funding, Four Communications’ annual earnings are expected to grow significantly this year.
"It is a great example of a London-based business that is seizing every opportunity to grow not just in the city, but internationally, to remain a market leader within the communications industry."