Breaking this down for those who arent aware, 是 means a few things:
- Yes/right (affirmative response)
- being/to be (verb)
So you can use it in a few ways, like
我是吃饭 to mean "I am eating rice" OR
是的 to mean "yes it is"
It just change to soft tone like 0, since it is on the end of the sentence. And indeed it is not 4, if it is 4, eng would be much more emphasized. you can check the online voice dictionary, word 剩 sheng4 is different from 生 sheng 0.