The default psake build file of default.ps1 is completely ambiguous without inspecting its contents. Rake has a Rakefile, bundle has a Gemfile etc. so should psake have a Psakefile.ps1 ?

Given that one of benefits of build automation is discoverability it would make sense that the default build file should be self-evident, or at very least suggest what it's intent is.

Assuming this is an accepted (and is a good idea) I would propose the following;
version 4.y

1. Change the default behaviour to load default.ps1. If it does not exist use psakefile.ps1. This would maintain backwards compatibility until a major version is released.
2. Add a deprecation warning if loading default.ps1.
3. Update documentation with deprecation and to say that version 4.y prefers psakefile.ps1

version 5.0

1. Remove support and documentation for default.ps1