Reduce hard voltage drops on mains?
You have no location information in your question or user profile but I'm assuming that you are in Europe.
4 V on a 240 V supply, which is 230 V nominal if you are in Europe, is nothing to be concerned about. European supply is supposed to be maintained at 230 V +10%/-6% so 216 V to 253 V.
It is more likely that you are experiencing a larger voltage drop than you can measure with your test equipment because it lasts &lt; 1 s. A multimeter may smooth or average out several readings and won't catch the dip in voltage.
You shouldn't be seeing this problem. It is caused by the surge current on motor start-up causing a voltage drop on either a long supply line, a faulty (high-resistance) connection or overloaded supply transformer. If you are satisfied that your building wiring is OK then you should raise the matter with your supply company and request that they monitor your supply and rectify it if found defective.