This graphics has been realized with the help of the following Octave script:

order = 4;
pointNb = 100;
axesLength = 1.25;
textOffset = 0.05;

angle = linspace(pi/2, 3*pi/2, pointNb);
poleAngles = pi/2 + pi * [1:order]/order - pi/(2*order);
poles = cos(poleAngles) + i * sin(poleAngles);

figure(1);
plot(cos(angle), sin(angle));
hold on;
plot([-axesLength, axesLength-1], [0, 0], 'k');
plot([0, 0], [-axesLength, axesLength], 'k');
plot(poles, 'xr');
hold off;
axis("equal", "off");
text(axesLength-1+textOffset, textOffset, "re(s)");
text(textOffset, axesLength+textOffset, "im(s)");
print -dsvg butterworth_poles_4.svg

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