When I changed them a piece of gunk dropped in and now Ive got a nice little misfire in number 4.

 

I was going to say that I don't think it would matter much if the spark plugs were in the front when it comes to crap falling in...

But then again, with the opening in the back, it means that, even if you took off the cowl, part of the Fit's sheet metal is above the spark plug holes. That is usually enough to prevent most random debris from falling in.

 

Even though its going to take work, take that plug back out and check it for damage. If you want to test where the misfire is coming from, swap the plug in cyl 4 with cyl 3 and swap the coil in cyl 4 with cyl 2. If the misfire moves to 3 it was a bad plug and if it moves to 2 its a bad coil. Better to do this simple test than to guess something damaged the inside of the motor.