Captivate 4 was based on Action Script 2 and Captivate 5 onwards versions are Action Script 3 based, it is always recommened to open a project in the next higher version, if you are upgrading your project.
However, there is an option in Captivate 5.5 to open .cp files, when we click on File > Open.
You can make a back up of your Original Captivate 4 project and then Launch Captivate 5.5 and click on file> open > borwse that project and then open it.
P.S.: Please make a back up of your original Captivate 4 project and then open another copy in Captivate 5.5, as a project once opened in Captivate 5.5 and saved, cannot be opened in Captivate 4 again.
Varun, in CP4 one had the choice between AS2 and AS3, it was not only AS3.
Thank you very much for your response. I was unable to purchase an upgrade
to 5.5 and settled for a 4 upgrade to 6 which is currently on order. Do you
know whether the same option to read my 4 projects in 6 will also apply,
and are there any other issues related to the migration of my projects,
particularly those bearing on AS2 vs AS3?
Yes, If you have Captivate 6, then it can also open .cp files created in Captivate 4, the procedure is same.
Earlier versions use RTF to render text while Adobe Captivate 5 or Later use ATE , each of which has a different scaling for font sizes. Therefore, the font sizes may vary when projects are upgraded to Adobe Captivate 5 or later.