By Lars Mucklejohn Monzo is launching a secondary share sale that is set to value the digital challenger bank at £4.5bn, City AM has learned. Hundreds of employees are expected to sell tens of millions of pounds-worth of stock as part of a deal backed by Monzo’s existing investors, it is understood. The sale is said to involve existing investors, including StepStone Group and Singapore’s sovereign wealth fund GIC, which have agreed to buy stock from employees. The bank is planning to announce the share sale on Friday, it is understood. Monzo declined to comment on Thursday. A £4.5bn valuation …