That's not the full scenario, UEFA will allow a maximum of five English teams in the CL.There is a crazy scenario where 4th can miss out on CL, I think arsenal have to win the EL
So, you would need Arsenal to win the EL, outside of top five in prem for CL qualification. Then Chelsea to win the CL, outside of top five in prem for CL qualification, then 4th place would get EL.