I guess buses on a 4 week trip where you visit Venezuela, Peru and maybe Chile is impossible without sitting in those buses all day.
But I agree with Niels that when maybe chosing 1 or 2 countries close together it is possible to only use buses and fly directly from London to some city in South America. There must be some direct flights from London. There even are from Amsterdam directly to Lima and Sao Paulo for example.
From Santiago to Lima in about a month is very well possible and although I think the Angel Falls are worth it, it's a shame on both money and time, let alone the environmental aspects.
On the other hand, I do understand that when going over there you want to see as many highlights as possible, maybe you won't go there anymore afterwards.
Agree with the airlines mentioned though, LAN (both Chile and Peru) are great and also TAM (Brazilian based) and Aerolinas Argentinas are among the best airlines. Avianca from Colomba could be another one, but unfortunately I don't know by head what airlines serve exactly what routes.
I do know that domestic flights in Venezuela have a poor safety record!
