On femurs, anything up to 6cm is low risk. This comes straight from Paley. 6-8cm is considered "medium" risk, according to him. If you're starting at 180 cm, however, up to 8cm is (probably) also low risk. You lengthening 6-8 cm is less risky than someone who's 170cm doing the same thing.
I would not do this surgery for just 4.5cm, imo.