I know this thread is a few months old, but thought I'd point out that the UC4 EVENT/FILE object type has a FILE_STABLE option in the Event settings. I've used this feature with a wildcard too, and it validates that all of the files that are selected are stable before it will be satisfied. You can customize how long it waits between stability checks, but the smallest wait is one minute, which is what I've been using.
You could precede your FTP step with one of these objects. But of course this will only work for checking local files, not remote files. If you need to check remote files, then I would think Alain's recommendation would be required.