$\text{Đáp án + Giải thích các bước giải:}$
`(x-4)/(2017)+(x-3)/(2018)+(x-2)/(2019)+(x-1)/(2020)=4`
`<=>((x-4)/(2017)-1)+((x-3)/(2018)-1)+((x-2)/(2019)-1)+((x-1)/(2020)-1)=0`
`<=>((x-4)/(2017)-(2017)/(2017))+((x-3)/(2018)-(2018)/(2018))+((x-2)/(2019)-(2019)/(2019))+((x-1)/(2020)-(2020)/(2020))=0`
`<=>(x-4-2017)/(2017)+(x-3-2018)/(2018)+(x-2-2019)/(2019)+(x-1-2020)/(2020)=0`
`<=>(x-2021)/(2017)+(x-2021)/(2018)+(x-2021)/(2019)+(x-2021)/(2020)=0`
`<=>(x-2021)((1)/(2017)+(1)/(2018)+(1)/(2019)+(1)/(2020))=0`
`<=>x-2021=0` `\text{. Vì}` `(1)/(2017)+(1)/(2018)+(1)/(2019)+(1)/(2020)\ne0`
`<=>x=2021`
`\text{Vậy}` `S={2021}`
