Originally Posted by
pjthomas
For Shimano MTB up to 9 speeds, all rear derailleurs are interchangable. The difference isn't the number of gears, it's the build quality and materials. The RD-M4000 is really a previous Deore rebranded, IIRC RD-M591. There is a big difference between Altus and Alivio.
That said shifters have a more significant impact on shifting than the rear derailleur. Swapping out the right shifter and the cassette would greatly improve shifting.
You had upgraded your old Altus C201 to the Alivio M4000. Yes, most definitely that is a significant difference.
The OP is talking about upgrading a Altus M370, which is a significant upgrade to your old C201. The M4000 Alivio while is an upgrade to the M370, it's not the same significance that a Altus C201 to the Alivio M4000 is.
Both the Altus 370 and the Altus M2000 alike the Alivio M4000 has trickle down tech from Deore. But there is a difference, Deore build tech is better being that it's all metal. The Alivio M4000 and Altus M2000 uses plastic. Granted I haven't seen any problems with mine and it works fine, but for heavy mountain bike usage, metal might be better than plastic.
For a typical tourer who is not riding down mountain sides, none of this really matters, there just isn't a significant difference in quality between the newer Altus and Alivio's. What's even more interesting, is that Shimano no longer even has the Alivio on their web site. Only the updated Altus.
If the OP likes what he/she is spending money on and will make the ride more enjoyable. Then great, money well spent.