I think she's talking about if the Sims 4 is such a step up that it's hard to go back to the Sims 3, not if it'll cause any kind of problems from having both installed. So assuming it's about that:
I play TS4 for its beautiful graphics and Sims and to not be challenged. I play TS3 to be challenged and to have a lot of fun. I went back to TS3 once after an extremely long hiatus and had so much fun, more than I did in the TS4. TS3 accomplishes a lot of things TS4 fails at, but TS4 still has some superiority over TS3, such as game stability. So no, playing TS4 shouldn't (unless there's something TS4 does that ruins TS3 for you) ruin TS3 for you.
