4. Providers of online platforms shall handle complaints submitted through their internal complaint-handling system in a timely, non-discriminatory, diligent and non-arbitrary manner. Where a complaint contains sufficient grounds for the provider of the online platform to consider that its decision not to act upon the notice is unfounded or that the information to which the complaint relates is not illegal and is not incompatible with its terms and conditions, or contains information indicating that the complainant’s conduct does not warrant the measure taken, it shall reverse its decision referred to in paragraph 1 without undue delay.