Horsman will work with directors Mark Knight and Sally Moore, and report to group chief executive Nan Williams.
Horsman will be responsible for the agency’s corporate reputation work, specialist financial services programmes, litigation support and crisis management, which includes the agency’s data-driven Protect service.
He has about 20 years' experience advising large businesses and recently formed part of the Saudi Arabia-based team advising the $2tn IPO of oil giant Saudi Aramco.
Prior to that he held roles at the BBC, the Cabinet Office and the House of Commons, with agency experience at Edelman, FTI Consulting and Weber Shandwick.
"My experience tells me that the London and Gulf markets where Four operates are arguably the two most competitive and highest-quality agency environments in the world. I look forward to playing my part in leading and learning from my new colleagues and clients to build on Four’s tremendous track record," Horsman said.
Four Communications recently reorganised its business, reducing the number of practices from 17 to seven multi-service business units.
The agency’s corporate and financial business is reported £4.5m in annual fee income, and clients include Baillie Gifford, Sainsbury’s Bank and American Express.
Williams said: "We were impressed with James’ enthusiasm for our digital and data-led future and we are looking forward to working with him to bring innovation to some of our more traditional and highly regulated sectors."