You can get 2.4.0 (70904) directly from Amazon. The secret sauce is in modifying the URL for the current version. In this case, changing the 70964 directory to 70904 and the filename from KindleForPC-installer-2.6.70964.exe to KindleForPC-installer-2.4.70964.exe. Is thanking Amazon for not purging older versions so you can still the 2.4.0 version is appropriate?
Code:
https://kindleforpc.s3.us-east-1.amazonaws.com/70964/KindleForPC-installer-2.6.70964.exe
https://kindleforpc.s3.us-east-1.amazonaws.com/70904/KindleForPC-installer-2.4.70904.exe
KindleForPC-installer-2.4.70904.exe