Haarmaan is currently refusing to name the four UK lawyers due to "existing contractual obligations" but says all are drawn from "leading UK firms or Chambers".
The new London office will concentrate on company law, M&A work, capital market law, financial services and tax.
Wilhelm Haarmaan, one of Haarmaan's founding partners, says: "We are continuing the strategy adopted hitherto of expanding without merging. We find that clients are increasingly looking to us to support their cross-border activity.
"London is a major centre of competence for Anglo-Saxon law and therefore it is essential for us, a firm with global ambitions, to have a strong presence here."
The new practice will take Haarmaan's office count to 17 world-wide – six of them German and ten international. This year alone the firm has expanded into Shanghai, Singapore, Budapest and Gliwice