A person is guilty of burglary in the first degree when he knowingly enters or remains unlawfully in a dwelling with intent to commit a crime therein, and when, in effecting entry or while in the dwelling or in immediate flight therefrom, he or another participant in the crime:

1. Is armed with explosives or a deadly weapon; or

2. Causes physical injury to any person who is not a participant in the crime; or

3. Uses or threatens the immediate use of a dangerous instrument; or

4. Displays what appears to be a pistol, revolver, rifle, shotgun, machine gun or other firearm; except that in any prosecution under this subdivision, it is an affirmative defense that such pistol, revolver, rifle, shotgun, machine gun or other firearm was not a loaded weapon from which a shot, readily capable of producing death or other serious physical injury, could be discharged. Nothing contained in this subdivision shall constitute a defense to a prosecution for, or preclude a conviction of, burglary in the second degree, burglary in the third degree or any other crime.

Burglary in the first degree is a class B felony.