PIN numbers do not replace passwords. A password is still required. PINs allow you to have a very strong password with the convenience of being able to quickly unlock your device.
PIN numbers in Windows 10 are not really included to increase the security of that specific device. In order to configure a PIN number on a device you must first log into that device using your Microsoft account and then configure a PIN number - the PIN number only unlocks the device you've set that PIN number up on, meaning that if the PIN number is compromised the attacker only gains access to that machine, not to your other devices, not to your Microsoft account.
As per one of the moderators on the Microsoft Answers question "PIN makes Windows LESS, FAR, FAR, LESS secure":
- The PIN can only be used on the physical device it is set up on
- PIN login only allows 4 incorrect attempts before you're challenged
- After 1 more failure, you must restart the machine
- After going through the above process a couple of times the PIN is blocked
Essentially this is a convenience feature, allowing you to a use a very strong password for your account and a much easier password for unlocking your devices. Entering the PIN number also requires physical access to the device, so brute forcing it would be time consuming and require manual input.