was just reading through the contemporary popular music syllabus for the australian music examination board (where they use the acoustic piano, digital piano, and electronic organ). they have a simpler, yet probably great method for young children as well as older beginners.
www.ameb.edu.au/MiscPDFs/CPM_revKeybrd.pdfthey suggest other ideas, too, such as:
working on a well balanced posture
hand and finger positions that facilitate performance
learning about tempos(steady sense of time and rhythm)
appropriate dynamics
beginning reading skills
i like their sight-reading exams because they only focus on four bars (though for a four year old - it could be one or two)
you could make a huge octave keyboard with each of the notes being pieces of a puzzle or pillows that fit together. mess it all up. see if the child can put it together. show the pattern of two and three black notes first. let the child keep the puzzle.
coloring books are great, too! writing the numbers of the beats (1,2,3,4) gives them a chance to write their numbers - and of course, the letters ABCDEFG. later on in the coloring book (if homemade) you could ask them to write all the letters of A in the appropriate place on the keyboard - and so forth. then - have them find the notes on the keyboard.
seeing the patterns of notes seems like a good place to start - as well as getting comfortable hand position and just having fun playing. (even if they goof off a little).