For the 6 year the remote control car would be okay (just make it one that's not too complicated ...check the age on the box) or maybe anything Spiderman or Hulk ...
Ruth ... my Godson is almost 4 ... he's into stuff like flashlights (bought him a Fischer Price flashlight that flashes different colors) and tools ... those work benches that they bang the plastic pegs into, plastic wrenches, etc ...
An idea for the 4 and 6 may be "rescue heroes". (Probabl more for the 4 yr old though) Great positive influences. Also, simple leggo sets. Some follow Star Wars or other series