With your old stored files, you have to put 4.3 on the original computer where the files were uploaded, because they are still tied to it until you run the File Conversion Tool.
4.3 still won't play the files you've stored on hard drive or moved around.
Does 3.4 have the Backup Tool? That's another Sony genius idea that sends a code to some server in Sonyland and theoretically allows you to then back up the files if you have to do a system restore/operating system upgrade. Or something like that.
Here's the Google-cached FAQ, since Sony Insider is down at the moment.
Complicated enough?
http://74.125.47.132/search?q=cache:RLAQ8g4wUr4J:forums.sonyinsider.com/index.php%3Fshowtopic%3D3373+sonicstage+Backup+Tool&cd=3&hl=en&ct=clnk&gl=us&client=firefox-aSince Sony has discontinued ATRAC (the minidisc format), I don't know if the Backup Tool server is still online, but you might as well give it a try.
Then of course you have to back up the files.
I never used Backup Tool, but if your 3.4 with the original files has it, I'd use it before installing 4.3 on top of it. As you know, once you make a backup, you never need it, but if you don't make a backup....
I've had no trouble with any SonicStage upgrades since version 2, and neither did virtually anyone on minidisc.org. It was one of the few Sonic Stage non-nightmares. But perhaps because I started using SonicStage in its early versions, I've never exactly trusted it. So run the Backup Tool once you figure out exactly how, then upgrade on the original computer, then run the File Conversion Tool.
I now run the File Conversion Tool every time I upload a recording. It calculates how much it would take to do the entire library again--because it's unprotected, and it immediately wants to copy-protect everything--and gives you some appalling amount of hours needed. Then you uncheck the copy-protection box and it runs for a short while to convert what you've just uploaded.
You really have to wonder what the Sony software guys were imbibing. Especially since the hardware guys gave them such a nice device until they made it so user-unfriendly.