janet,
i agree with melissagray. family can be so supportive and provide lots of help and love. it would be a great deal easier for you if you had family close by. the key word here is *help*, help, and more help. there is no saying that it cannot be done, because with the will and strength and determination, it can!
you may also want to research medical schools that you may be interested in applying to and see if they have childcare programs on-campus or close by. for instance, the universtiy in which i attend has several programs and even an elementary school on campus! those kinds of factors can also be of great help...the only thing is it costs money, and if you have it or can get it by financial aide, go for it!
another possiblity that you may want to consider (if your willing to) is wait for your children to be a bit older (the youngest in grade school). it involves patience as you would have to sacrifice years to finally persue a career in medicine, but it is a possibility and plenty of moms do it!
sorry for the rambling, i wish you all and your 4 children all the luck!
annie