What formatting means: Think about the way a library is organized. If you just threw the books in randomly until the room was full then you would fit far more books, right? But you would never be able to find a particular book without searching through every single book one by one. Instead, libraries employ a sorting system (which takes up space!), and employ corridors and shelves (effectively empty space or space not used for books!) to make it possible to actually find a particular book in a timely manner.
You need to understand that the same philosophy goes into formatting a computer disk. Usually, it means 2 things:
- Adding spaces between the areas in memory where data will be written
- Adding data (such as a File Allocation Table or similar) that tells the operating system where files are stored.
What's in a GB?: Another factor at work is described in David Marshall's answer. Disk manufacturers tend to represent 1 GB as 1 billion bytes, when it is more typically represented as 2^30 bytes. Assuming this is the case for you, you really have 4e9 / 2^30 = 3.73 GB.
Bad sectors: another thing that happens when you format a drive is that the operating system looks for and removes so-called bad sectors. These are areas of the disk that the OS can't read/write to. It marks these sectors as bad while formatting, so that they are effectively quarantined and don't cause your data to get corrupted. This system works well if the disk has suffered a small amount of damage and the source of the damage is not recurring. The downside is that the overall available disk space goes down since more disk space is no longer being used. This is a potential reason why your available space on the drive has gone down.
Environmental factors: has anything else changed about the version of your operating system or the formatting tool itself? Any changes here could potentially cause the behaviour to change.
Conclusion: Out of your original 3.73 GB of space, the remaining 3.73 - 3.69 = 0.04 GB was probably lost due to the organization system and/or bad sectors. Different formatting systems will use up more or less of the space on your disk. Bad sectors are, generally speaking, not recoverable. Potential solutions to the problem include:
- Buying a new, bigger drive (most likely to succeed!)
- Ensuring that you're using the same version of formatting utility or operating system as you previously used (if applicable)
- Trying a different formatting utility
- Applying a different file system type to the drive (if the formatting utility allows it)