north vs south CA

Personally I would skip El Salvador or Honduras and replace one or the other with Panama. Panama is a beautiful country and there is more to see than the canal, although the canal is an incredible site and should be seen. If you enjoy history, a great book is "Path Between the Seas" by David McCullough. It is the bible on the building of the Panama Canal. It would be great for someone visiting Panama and the canal to have the perspective of all that took place to get the canal built. I just think the appreciation of the site and the country would be at a totally different level. There are some ruins in Honduras to see. I just think with only 4 weeks, even though that seems like a lot of time to some people, it really isn't. People spend months at a time just in Costa Rica and go away realizing they only scratched the surface. Even if you are not beach people, spending some time on the Islands around Bocas Del Toro is recommended. There is some great hiking around Bastamentos Island just across from Bocas, and you will love all the little restaraunts and bars along the water in Bocas.

Guatemala is the cultural highlight of the entire region, this should be your focus IMHO. At least 2-3 weeks.

Costa Rica offer more adventure, though its the height of rainy season and tropical depressions, be warned it will be a mess in October, little sun.

Since you don't want beach... you could do the whole time in Guat and not get bored. Of viably, Guat. with a hop over to Honduras for Copan and maybe one other spot. If you aren't into surfing or gangs, skip El Salvador. If you haven't already...check into visa requirements for Singaporeans. Belgium is easy...just show up at the border.

depends what you want, but the north option definately offers culture and ruins that the south option does not.

of course theres lots to do and see in Panama besides the canal.

Sorry to jump on, but is yellow fever vaccine required for Panama? I can't figure out where to go next but I don't want to get a vaccine right now. thanks!

I am not aware of Yellow Fever shots being needed in Panama. Malaria is more prevalent than CR, but mainly in the Darien Gap area and the mountains on the Caribe side near the border with CR.