You don't have to have 4 jacks but it is nice for doing a tire rotation (loosen the lugs before jacking up the car).
The imbalance you are getting can be completely avoided. After you have either the front half or the rear half up in the air and the jack stands in place, place the jack at a central location on the other end and lift from there. Otherwise you can possibly cause yourself (and the car) some serious harm from the car sliding off the jacks.
Usually you can find a solid frame area under the engine or the rear differential in the rear. Also, if you are only jacking up one end, its not a bad idea to use tire chalk-blocks in front of the tires on the other end preventing the car from moving when you lift the car.
One more tip, find or cut a 2/4 to fit inside or on top of the jack lifting point, my jack has a round bowl that rotates. I just cut some scrap wood to fit inside the bowl, that way the wood deforms rather than any metal on the car.
Last point, only lift on level surfaces.