Charaters/Bytes count and File size on Windows pro

Question

I have a txt file got generated through PHP script. File character count is shown correctly as 3999 bytes/characters when i checked through my script.

When i checked the same content by copy&paste into MS-Word, still it was showing 3999 characters/bytes(with spaces).

However, when i looked at the windows property of the same txt file, it shows the size as 4.17 KB (4,278 bytes).

I am just wondering, what could be the reason for such big margin of difference when i had looked at it. If someone can clarify this, it would be great.

Thanks in advance.