Is PM 4 the last version for Mac OS 9?

[Dr Dave]

I'm using PowerMail 5.01 on Mac OSX. 
I want to recommend it to people still on OS 9, (and do not have the
option of upgrading to OSX) but I need to know if they will get the same
experience that I'm having. It appears that PM 5.01 is only for Mac OS X. 
Is this true? What are the main differences between 5.01 & the version
for Mac OS 9? Has the database format changed as well?
Dave