There are probabably a hundred questions (and answers) which might help troubleshoot this problem. Here are some that come to mind:
Does the engine spin and not start?
If it spins, is the prop turning over slowly, at a normal speed, or at <relatively> high rpm?
Can you confirm spark at all 4 cylinders?
What is your starting technique?
Does the engine fire at all?
Which fuel injection system are you using?
Is this a new problem (i.e. a new engine installation) or is it a recent development on an already flying airplane?
