+1 Cams will tell you' its only going to fire when both valves are closed. Thats only important for EFI engines though, CDI engines are (AFAIK) all wasted spark, which means that one coil pack connects to the inside two cylinders and the other connects to the outside two.
The piston ring grooves hold the piston rings, without them you wouldn't have oil sealing or prevent the combustion process from just bypassing the piston into the sump
Piston ring posts are to locate the piston ring in 2 stroke motors so that the ring doesn't rotate around into the port area, the reason this is important is because if it does then the piston ring can open up into the port area then the next time the piston travels up (happens quite frequently) the piston ring will be snapped. In 4 strokes its a non issue because you don't have ports, so there aren't posts.
EDIT: I'm assuming you're asking about an I4 engine for the firing order, for a single its pretty simple and for a twin you just check the cams again.