What is the difference between 阻拦, 阻止, 阻挡, and 阻碍?
As a native speaker, I personally use them or hear people using them interchangeably. More specific distinctions are as following:

阻拦 means to intercept, which can be used to stop someone from doing something or stop something from happening.

阻止 means to stop ... from ..., which is usually used to stop someone from doing something.

阻挡 means to block, which is usually used when someone or something is physically blocking in the way.

阻碍 means to obstruct, which is usually used when a difficulty or obstacle is posing a 阻碍.