I live in Switzerland, so the four seasons would probably be great for September to mid November and March to Mid/End June, as well as on not so great summer days (which there are plenty of).
Mid gray flanell was one of the candidates for the third and forth pair, but I am not sure, if they look good with just a sweater + shirt.
Summer weight I wasnt really sure, but people in the HY thread recommended exactly the pair you've mentioned.
Let's say you would go with the mid gray flanell and the light gray summer weight, what other two pair of pants would you recommend?
I would probably get a brown next, your choice as to worsted, flannel, or summer weight.