The latest software for 4000 and 6000 processors is Dynamic C 10.72E. There is no DC10.73.
Please see link below to download latest; https://hub.digi.com/support/products/dynamic-c/?path=/support/asset/dynamic-c-1072e-new-release/
All documentation and older releases can be found at this link; https://hub.digi.com/support/products/dynamic-c/
Github has all patches that have been released after the official releases. https://hub.digi.com/support/products/dynamic-c/?path=/support/asset/dcrabbit-9-on-github/