I don't think there are "the" 4 causes, as in 4 causes widely agreed to be the most important: there were many more causes. But here are 4 causes that I find important:
-political importance and prestige: countries wanted to be better than other countries...
-economical: countries sought wealth, for example natural resouces
-religious: countries sought favors with the pope by converting the "pagans"
- curiousity in the Age of Exploration: the will to discover our planet
