Tetanus Possibility Rusty Blade?

How possible is it that I will come down with tetanus? My quarter rolled down into a hole in my car (The one where the seatbelt receivers come out of) and I reached down there to get it. I didn't find the quarter, but I did find a rusty blade. It wasn't completely rusted, but I was cut. The cut was very shallow, maybe just the first layer. It's roughly .5 cm across also. My last tdap booster was maybe 4 years ago. I cleaned the wound thoroughly and I let it bleed out a little bit. I cleaned it as soon as I noticed I was bleeding (Which was immediately). My parents said I would be fine and I don't need to see a doctor, but how likely is it I would come down with tetanus?