Flash disk capacity turns from 32GB to 4MB

It's a Counterfeit/Fake Memory Drive:

Quote from eBay from a paper trying to make people aware of the issue:

Fake memory drives have cracked hardware that will display fake/oversize capacity when you connect it to your computer. It may appear you can load this capacity on the drive, but as soon as you go over its real capacity the files will become corrupted and read strange file names. Fake drives usually are stated as having samsung memory. Please check ALL negative feedback of sellers and if people report counterfeit/fake usb drives then don't buy from seller!

http://www.ebay.com.au/gds/Fake-USB-DRIVES-From-China-HK-/10000000007355216/g.html

I suggest that you start by checking your drive with this tool:

Fake Flash Test

Here is a little documentation from the developer from his blog.

It might not be counterfeit, but have a messed up partition table. Doing this could help even if it is counterfeit, as even counterfeits are usually larger than 4MB. I recently had this happen with an 8GB flash drive of mine that I had configured as a boot disk and was showing up as 2-8MB (I forget precisely how many).

You need a partition manager that can rewrite the whole partition table. The Windows disk manager couldn't cut it. I used Mini-Tool's free version: http://www.partitionwizard.com/free-partition-manager.html

If you do get it to show up as 32GB again, try putting some large files on it (pretty much fill it up) then copy them back and verify their integrity.