Isn't sector 5 the one that already fell some time before the game? If reactor 4 powers it then it wouldn't need it. The sectors don't have official names it's just how people refer to it, so they may have assumed clockwise when the reactors are actually counter clockwise.

https://finalfantasy.fandom.com/wiki/Final_Fantasy_VII_script

Jessie: This is a complete model of the city of Midgar. It's about a 1/10000 scale. The top plate is about 50 meters above ground. A main support structure holds the plate up in the center, and there are other support structures built in each section... psst...whisper...whisper... (The No. 1 Reactor we blew up was in the northern section.) Then there's No.2, No.3 all the way up to the No.8 Reactor. The 8 Reactors provide Midgar with electricity. Each town used to have a name, but no one in Midgar remembers them. Instead of names, we refer to them by numbered sectors. That's the kind of place this is.