K.B.
You will need some sort of gaming system to play Guitar Hero; options are Wii, Playstation or X-Box. Guitar Hero is going to be hard for the 4 yr old and the 6 yr old will struggle with it also.
My suggestion is start with the Wii and get an age apporiate game and then in a few years get Guitar Hero. My daughter is 4 yrs old and we do have two games that she plays on the Wii and she kind of plays along with the mario cart and sports ones but they still are a little abover her age (I have seen kids her age do the games easliy but they spend everyday on the Wii where as my daughter gets to use it 1-2 times a week, for now). We do have Guitar Hero for Playstation 2 and LOVE it, my daughter watches and will fake play but there is no way she could actaully do it AND half the songs I would never let any one under 12 and some songs under 16 hear.