Fitting a 9-speed 11-50T cassette on MTB: which derailleur? I'm trying to understand if its possible to fit a 9-speed 11-50T cassette (found one on Ali) on my MTB. The plan is to go 1×9. The problem is that I can't seem to understand the if a specific derailleur can handle that.

The rear derailleur in mind is Shimano M4000 long cage. 
Should I use a hanger extension? Will 11-42T be better for that?

Thanks for the help
https://bike.shimano.com/en-EU/product/component/alivio-m4000/RD-M4000.html   says that derailleur will handle


Low sprocket_Max.    36T
Low sprocket_Min.    32T


ANSWER: You cannot run a 50 tooth cog with that derailleur.

You might be able to use a hanger extension, but that will make the small gears even worse.  Wolf Tooth's Roadlink allows up to 40 tooth, and Wolf Tooth's Tanpan says 44 or 45 tooth is the maximum cassette size supported.  Note these are for 10 and 11 speed.

If you're absolutely stuck on a 50 tooth rear cog, then you're looking at a SRAM 12 speed Eagle setup, which will not be cheap.  Cassette and shifter must change, and its very likely your freehub won't fit the cassette, and then the OLD will be too large to fit in your frame.  Not a good plan.