If she is already playing with Lego, duplo might be a step backwards. We didn't start accumulating any Lego until dd had just turned 4. At almost 4.5 she can now make some of the trucks in the city range almost by herself (she needs guidance, and like you mentioned, some things don't entirely click together for her).
Maybe get the duplo between your dd and her sister if you are concerned and hold off on Lego a little while. There is a new/dedicated range of Lego for young builders due to hit Australia in January. Looks to be some interesting sets! There is also disney princess stuff coming out in Lego rather than just duplo.

There is a Lego/mega bloks thread on here that has a lot of discussion about sets for kids at different ages. Mostly seems to be aimed at the 4-5 year olds given those of is crazy enough to be as in to Lego as our kids seen to have kids that age! Can't post a link though cos I'm on my phone