Just copy your runtime to another folder then install v4.2 as if somit goes wrong can put runtime back over, i just copy runtime over then rename v4.0 or 4.1 depending what v4 you got.
I found that when  installed 4.2 on computer without Internet connection could not get her work.
Had install on comp with open internet then transfer other comp, though will say never update the v4 as if i want use 3 v4s in a scene, i use v4.0 as less memory then v4.1 bit more memory then v2 with every thing loaded and much more memory.
Just copy ya runtime so if goes screwy  just copy back over.