Ciao, Those four expressions have similar usages but with slightly different nuance. 바탕 means background, so it refers more to the situation when something derived from a certain background. Which could be built upon the background already given. 토대 would be more suitable when you are describing the basic and substantial element of something. It means substructure ( for example of a building or sculpture ) and it is used metaphorically. 기초 means basic. It can be used to describe the initial motivation or starting point of something. 기반 means base. So you can use it to describe what smt is based on in general. I hope it helped :)