If an English side wins either competition it has no bearing on 4th.
If United win the Europa and finish in the top 4 - they will be in pot one of the draw rather than pot 2.
If City win the CL - They will be in pot one as UCL winners, and the spot for the English champions moves to another country
If Arsenal win the Europa and finish outside the top 4 - they will go into the Champions League in pot one and we will have 5 teams in it next season.
If Chelsea win the CL and finish in the top 4 - they go into pot one as the holders, and City will remain in pot one as Champions
If Chelsea win the CL and finish 5th, they still go in as winners.
Worst case scenario is that Chelsea and Arsenal win the respective competitions and then both finish outside top 4. Say City, United, Leicester and Liverpool finish top 4.
As per the UEFA rules is concerned maximum of 5 clubs can represent England. If the winners of champions and Europa league happen to finish outside the top four then the top 3 would get in the champions league and the fourth would be a Europa league spot.
This would mean Chelsea and Arsenal finishing outside the top 4.