USB stick reformat

Nothing; that's perfectly normal as the partiton and format leaves file structure settings, .Trash and other files required for OS X to read and write to the stick, which take up some of the memory space.

Reformat it.

1. Click on the parent drive (4.01 GB CHIPBNK ...)
2. go to the "erase" tab
3. Select either:
- "MS DOS" for Mac & PC
- "Mac OS extended (Journaled)" for the faster Mac only option
4. Give it a name
5. Click erase

If you want to attach extra security, you could create a separate "disk image" by going to "File > New > Blank Disk Image", making it slightly under 4GB (trail & error) and adding security options. Save that image to your USB.

As for the size of the drive:
The size will always be the same, but the usable data may change depending on formatting, security and partition map settings.
Another thing is that there is a difference between gigabits and gigabytes; goes up in 10s (100, 500, 1000) while the other goes up in exponents (256, 512, 1024, 2048). This causes misconception. USB producers will tell you that 1000 = 1GB while MAC OSX will tell you that 1024 = 1GB; this allows producers to make it seem that you are paying more data.
I'm sure someone can explain this better...

To illustrate my point about the files used for formatting etc., go to Disk Utility, highlight the HD (leftmost one with the disk info) and read the size at the bottom right. Then highlight Macintosh HD (which is the partition) and read the size of that.

The difference will almost certainly be somewhere around 350 to 400 MB. As the same formatting etc files are used for any drive, the difference you show is perfectly normal.

There are all manner of disk structures required to keep track of files and folders, and those structures take up part of the drive's space.