"The first series had Phoebe Waller-Bridge as the head writer, while Emerald Fennell took over for the second series. Subsequently, Suzanne Heathcote was the head writer for series three and Laura Neal will follow through as series four's head writer." (Wiki)
For me, series 1 (Waller-Bridge) was outstanding, series 2 was still good, but series 3 left me bored of both Villanelle & Eve. Carolyn & Konstantin were much better value & Fiona Shaw (Carolyn) was my highlight & kept me watching.