Shifting down after 4th gear

[Paulus]

I've wondered that too. I'm not sure how to check that. I've been wondering if I've done something really daft like not put the wheel in straight. I haven't done the brakes yet so I may have missed something. The thing is the wheel and cassette are from another bike and worked well on that as far as I can remember; that's why I pinched them...

It could be an indexing problem that may need tweaking.
I have in the past put the same freewheel on a different wheel and needed to tweak the indexing.

 

[Big John]

When you say you've checked the hanger alignment have you used the alignment tool and if so are you using it as per the instructions? The ones I've used aren't as simple to use as some folks think. Someone has already mentioned shifter/rear mech compatibility and I discovered this first hand when I couldn't get the gears to shift properly....it was close but definitely not 100%. The boss took one look and told me a SRAM shifter and Shimano rear mech wouldn't work. We switched the shifter and hey presto it worked fine. Easy to do when you've got loads of parts knocking around 😉

 

[Erasmus]

I know Shimano compatibility charts have been mentioned, but is that what your groupset is? Only reason I ask is that I had a similar problem to this with a 9sp campag set up, and it turned out I'd bolted the cable to the wrong side of the RD clamp. It looked that that was how it was meant to go, but instructions said otherwise, and moving it totally solved the problem as it minutely affect the cable pull.

 

[Ajax Bay]

Seriously wondering about swapping out the freehub and rebuilding as an 8 spd bike. I'm guessing an 8 spd mech wouldn't work on a 9 spd cassette?

No freehub swap required (and no spacers): freehub does 8,9 and 10sp (latter needs 1mm spacer).
Yes a '8sp' RD will work as well as a '9sp'. The shift ratio is the same: the shifter delivers the cable pull (less for 9sp than 8sp).

 

[Andy in Germany]

No freehub swap required (and no spacers): freehub does 8,9 and 10sp (latter needs 1mm spacer).
Yes a '8sp' RD will work as well as a '9sp'. The shift ratio is the same: the shifter delivers the cable pull (less for 9sp than 8sp).

Thanks, that means I can change it if needed. It also helps to be sure as I'm using used parts and I wouldn't be surprised to find a mech predated 9spd cassettes.

 

[Milkfloat]

You have tried everything else (except new chain), but I would go with what @Erasmus mentioned. The routing into the derailleur can be crucial, as can the routing under the bottom bracket, I have had problems on Shimano with that.