Sure a pin isn't as secure as a full password but we are talking about Windows logon here - It's there to protect against people casually trying to get into the PC, if you are determined enough it'll be easy enough to bypass a Windows login password and unless disk encryption is set up, even easier to access files stored on the system. As far as I'm aware, if you enter the pin wrong 5 times it will lock you out for 30 seconds, so if you factor that in, trying to brute force the pin is totally impractical. It's also possible to set up a total account lockout if required which will prevent an account from logging on if the password/PIN is entered incorrectly n times. If a 4 digit pin is enough to secure my debit card, it's enough to stop someone casually logging onto my PC.
On regular PCs I still use a full password since I find it's quick enough to type sitting at a keyboard (pretty much muscle memory) however I use a pin on my Windows 8.1 tablet (soon to be upgraded to Windows 10) - On a tablet a pin is nice and quick to enter, having to open a keyboard and type a password on an on screen keyboard while pressing different buttons to switch between alphanumeric keys and symbols is a total pain on a tablet.