Determine the value of the number ABCD

Paddy is smashing a few at the local until everything is forgotten. The bartender who is also a family friend continually tells him he's had enough and to go home.

Finally after several last calls, Paddy declares "I'm going home", promptly falls off his high bar stool and drags himself to the door.

He hails a cab while face down on the curb, manages to open the door and drag himself from his sprawled position into the backseat. The cabby drives him home with Paddy singing nonsensical music to himself the whole way. Paddy rolls out of the cab manages to drunkenly flop his way across the lawn, gets the front door half open and passes out.

The next day because the bartender is also a good friend he checks on paddy, and seeing him lying on his back in the doorway says, "Paddy, you were drunk last night weren't you?". Paddy replies, "Yes, but I didn't think I was that drunk, how did you know?"

To which the bartender replies, "You left your wheelchair at the bar".