举个最典型的例子
Kate has gone to Korea for two years.
和
Kate has been to Korea for two years.
前一句的意思是,KATE已经在韩国待了两年,现在回没回来还不知道.
后一句的意思是,KATE两年前去过韩国,现在她已经回来了.
是动词 been和gone的区别.
同理,
这句话如果改成
It has been 4 years since I began to smoke. --就是 “我抽烟4年了”
此句是begin to smoke和smoked的差别.