It is usually better to design these things instead of guessing.

Let’s suppose we want to keep the voltage drop to 0.2 V over 4 m of cable, and let’s say the laptop uses a maximum of 5 A. (Most laptop PSUs’ output varies by at least that much in normal use.) As for the maximum current consumption, check your power brick’s maximum output and use that instead of 5 A.

(The target of a maximum 0.2 V drop is a conservative estimate – it’s only 1% of the typical laptop PSU output! In other words most laptops could probably tolerate a considerably greater voltage drop than that. And the 5 A current draw is quite a high estimate especially for many smaller laptops. On the other hand, your power brick’s output might already be a little bit low, and other things might be not right on spec, and we’re adding some splices too… so a conservative estimate isn’t unreasonable here. See the comment discussion.)

The resistive drop in a conductor is simply E=IxR, where E is the voltage drop in volts, I is the current in amperes, and R is the resistance in ohms.

Then R=E / I=0.2 V / 5 A=0.04 ohms. You need your round-trip (ie counting both sides) cable resistance to be less than this.

Each side contributes half, so you need the R of each side to be half of this: 0.02 ohms for 4 m of cable.

Most wire tables list wire R in ohms/1000 ft or ohms/km. 1000 m / 4 m=250, so multiply both sides by 250 to get 5 ohms per 1 km of wire. We want each side of the cable to have R of no more than that.

Consulting a standard wire table, it appears that cable with each conductor using 2mm diameter wire will be close enough (R only a few percent high). In AWG the closest standard is 12 gauge.

That will be some fairly thick cable (in the US we use 12 ga. conductors for 20 A outlets and it is also the heaviest extension cord in anything like common use). And do remember that your splices at each end will likely add some non-insignificant resistance too.

4 m of very thick extension cord does not seem to me to be very convenient to just take down from a shelf and put back up frequently.

If your laptop uses significantly less current than that, or if you think you can tolerate a larger voltage drop, then you can use thinner cable. You now have the formulas and the table to find those numbers.

But I agree with the others – I’d extend the AC.