Make sure the bator is in a draft free room with a steady temp, away from windows (direct sunlight shining down on it) and out from under lamps that may be left on long enough to throw extra heat down on it. These things help cut down on fluctuations. I'd also recommend using at least 2 thermometers that have been checked for accuracy.
It's a lot nicer to have a thermometer to the tenths, but it is not dire, (despite what some people think) as long as your thermometer is accurate (never trust an unchecked thermometer even brand new) and stays between 37-38 you should be good. I have two thermometers iin my bator that read to the tenths and one that doesn't. I'm just happy if I can keep the darn incubator within 1 degree of the required temp.
The best way is to use a glass bulb thermometer that has been tested by taking a glass of ice and putting in cold water, let it sit for three minutes and place the bulb thermometer in it and let it register. It should register 32F or.......0C. Once you know how accurate (or inaccurate) that is you can place them both in the bator side by side and compare the readings.
What kind of eggs are you hatching? And no, I check at least every 20 minutes and am up every 1-2 hours at night to make sure I have no temp spikes...lol
Oh, yeah EE. I thought maybe you were hatching ducks when you said 4 weeks...lol
