Do not, by any means, consider this as a casual exercise. Ideally, you should have already decided on your list of universities before you take the GRE and so you can choose 4 from that list. However, we know that since GRE scores are a big factor in selecting the universities, most people do not have such a list. The straightforward solution is to have multiple lists. Make 3 lists of universities - one that you really want to apply to provided you get a great GRE score, one that you will apply to if you get average scores and the last fall back list which you use only if you screw up the test very badly. The US News and PhDs.org rankings explained above should be instrumental in helping you create these 3 lists. Doing it this way means extra work but it also means that you use test fees more efficiently by applying to the right universities.