Colombia is perfect for a four week trip. Fly to Bogota, then go to Villa de Leyva, one of the most beautiful towns in all of South America. Then go to San Gil white water rafting and paragliding. Then go cross country to Cartagena (a stunning city) via Mompox, which is like something from 1940s Mississippi... After that, you can do a 6-day jungle trek to the Ciudad Perdida (1,000 year old lost city in the mountains) which is like something from Peru, only spookier.
If you stay on the main tourist trails (which aren't over run like the rest of South America) there will be no safety issues at all. Plus, Colombians are easily the most friendly people you will meet in South America, and it's dirt cheap.
I couldn't recommend it enough. A brilliant country to go to if you only have the four weeks....