what nemasys said is correct, with one exception, how the did the test.

if you are seriously considering the car, have them test it again after its been back together and run a few miles starting on the #4 cylinder. if it is the same low cylinder nemasys is correct, if the number is high, they might have allowed the cylinder to dry rev too many times during the last test. A better test would be a cylinder leak down, and see where the problem comes out..

a slight variation (10-15 lbs) is kind of common on engines with some mileage. one low cylinder indicates problem. putting oil (depending on the amount of oil) only cures ring and piston gap issues, a lot of oil will change the compression ratio and raise the number falsely. low compression due to valves can be solved with a lot of oil as well.. or end cylinders could be a head gasket..

if two cylinders are low, and beside each other it typically indicates a bad head gasket. plus you would have a power issue.