@Kickass
If you have a digital refractometer it is important that you clean the sample from solids in suspension, e.g. with a paper coffee filter.
If you have an analogic refractometer, the reading is dependent from the quality of the light. Ideally you should use it with natural light (day light), if you use a different light source (a LED lamp for instance) you should at least calibrate again with the new light source.
Also, in four days your sample might have undergone some evaporation and its density might have increased.
Yes it's a bit frustrating that the results are not always so "spot on" as with a hydrometer, but on the other hand it's more practical, you don't have to make your eyes bleed trying to read the hydrometer ;-) you waste less wort and it's much more difficult to break a refractometer than a hydrometer.
My hydrometer is so stupid that it hasn't the ,005 values marked with a different line. I have to count whether I see 8 or 9 little lines from the first round value above, which isn't so easy, especially through a plastic cylinder which is not perfectly transparent not uniform. Plus, I must take a reading on the upper part of the meniscus, and often I cannot see the meniscus. For easiest reading one must fill the cylinder so that the liquid line on the hydrometer is seen directly and not through the plastic.