I apologize, we forgot to add 3.3.3 to the list of convertable versions in the latest 4.x release. I suggest filing a ticket if you must convert to the latest 4 version really soon, if not you can wait for the 4.3 release where this will be possible.
Strange. That's normally the type of message you'd expect when trying to open a project in an earlier version of the Modeler.
Looking at the 3.3.3 release, it seems to contain some changes that are also in version 4.2.2, but not in 4.1.0. I wonder whether that is the issue.
Have you tried updating:
(a) from v2.5 directly to v4.1.0
(b) from 3.3.3 to v4.2.2
This makes sense as your 3.3.3 version is much newer than the 4.1.0 release. Normally you'd need a later 4.x build to convert this project. However, in this case we also forgot to add 3.3.3 to the list of convertable versions. I suggest filing a ticket if you must convert to the latest 4 version.