For your first question. You can plot an inequality with two variables like this
q = 1;
p4 = 2;
[p1, p2] = meshgrid(linspace(0,1,100));
s = q*(2*p1+p2+p4-1) <= 2*sqrt((q*p2 + (1-q)/4)*(q*p4 + (1-q)/4));
contourf(p1,p2,s+0, 'LineColor', 'flat');
xlabel('p1');
ylabel('p2');
For the second question, I am not sure how you can visualize it. With the help of slice(), it is somewhat possible to visualize 3 variables, but four variables cannot be visualized in a single graphic. Plotting several 2D plots by fixing the values of the other two variables is one of the ways.
