Please don't discount all the other influences,a 4 year old needs,that are not computer related.
Den building (construction) cooking (science) any park or open space (exercise)
There are some great,basic science sets,which are plenty simple enough for a youngster,with circuit design,crystal growing and many others... you want to see a group of kids faces,when you use an air-bed pump,to make water boil,after it has cooled...
I have had some "illuminating" conversations,over the years,with parents who have left their kids at ours,for a sleep-over,and asked for them not to be allowed any climbing or shooting...
This is perfectly acceptable,and was always respected,but then you learn that the same parents,allow them 20+ hours on a PC at weekend,shooting prostitutes and learning foul language...
We risk a generation of adults,with square eyes and fast thumbs,no legs,and a tube from their outlets,to the nearest drain...