I have a Shimano 7 speed rear derailleur which used to work properly. Lately I observed I was unable to shift down to gear 7 from 6 but let the problem be. The derailleur would move very little (there wasn't even any grinding sound against sprockets when shifting).
Now the gear won't even shift from 4 to 5. Derailleur works fine till gear 4 but as soon as I shift to gear 5, the movement is miniscule. I checked the wire and it was all slack (literally hanging with 0 tension) in this position. Clicking to shift down to 6 or 7 doesn't even produce a clicking noise and there is no effect on wire/ derailleur.
I have read about wire tightening (per this question) but those steps require chain to be on lowest cog. My wire goes all limp in gear 5 and chain is on fourth sprocket. Can I undertake the tightening procedure in this situation? How do I gauge requisite wire tension in this case?