The UK arm of the Netherlands-based company was launched two years ago after Royal Mail's 350-year monopoly ended. Letter-delivery was opened up to full competition this year.
Four's hire spells the end of a two-year relationship with Financial Dynamics.
Account manager Lucy Bennett said the agency would promote TNT Post's services to the business sector via national, regional and specialist titles.
The Four team, headed by CEO Nan Williams, reports to TNT Post CEO Nick Wells, although its day-to-day contact is head of marcoms Simon Dolph.
It secured the £100,000 brief after a competitive pitch against a small number of undisclosed agencies. FD declined to comment on whether it had repitched.
TNT Post has won ‘downstream access' contracts with Barclays, LloydsTSB, BSkyB and Telewest - it collects
and sorts all mail from the companies before final delivery by Royal Mail. The firm is currently working to cut Royal Mail out of the loop altogether.
It claims to deliver over 1.2 billion items of mail a year, making it the biggest competitor to Royal Mail in terms of volume delivered. TNT Post and its competitors have reduced Royal Mail's market share to around 98 per cent.
TNT Post teamed up with mail-order firm JD Williams' courier network in September, vowing to become the leader in residential delivery of packets and parcels within three years.
It also delivers 2.5 billion items of unsolicited flyers a year to UK homes.
ALSO... Four has taken on Weber Shandwick's ex-HR director Liz Fraser. Her main remit as head of HR is the recruitment, development and retention of staff.
Fraser was a founding director of Granard Communications in the 1980s before it was bought by Saatchi & Saatchi. She was European HR director of Edelman Worldwide before joining WS.