According to the online help for AMD RAIDXpert for SB850 (which I'm using), you should be able to migrate a 3-drive RAID 0 to a 4-drive RAID 0. From the OLH:
To migrate a logical drive:
1. Click the Logical Drive View icon in Tree View.
2. Click the Logical Drive icon of the logical drive you want to migrate.
3. Click the Migration tab in Management View.
4. From the Migrate RAID Level to dropdown menu, choose the target RAID level for your logical drive.
5. If your intended action requires additional physical drives, click a free physical drive to select it.
Available drives have a black frame. Selected drives have a red frame.
6. Click the Start Now button.
You can monitor Migration progress on the Logical Drive–Migration tab. Click
the respective buttons to pause and resume the Migration.
Now, that said, I haven't said anything about the wisdom of using a 4-disk RAID 0 array. You stated that you are using HDDs and not SSDs, so I am going to have to take it on faith that you have a data backup plan in place. Otherwise, you would obviously be setting yourself up for some serious pain - and in fact, even with a backup plan it will be painful to restore 4 drives' worth of data when one of your drives fails.