Which is usually a four digit code, since you have to type it in so often? That's about 10,000 combinations. It's trivial to go through all the combinations. Even if it took a second per combination, it'd be cracked under three hours.
A more sensible way to encrypt a device is to use a separate long code that only has to be typed in at boot. Using a screen lock as an encryption key is just not effective.
Someone can probably manually access this in about 8 hours. 4 seconds / attempt = 7,200 guesses ~72% chance to break in. (Assuming there is no built in lockout based on failed attempts.)
A more sensible way to encrypt a device is to use a separate long code that only has to be typed in at boot. Using a screen lock as an encryption key is just not effective.