1995 Galant won't start

We have a 1995 Galant. We have replaced the fuel pump and we are getting fuel to the engine. We put in new spark plugs and are getting spark from the distributor to the spark plugs. We check the timing and timing indicator dot aligns up with the "scoremark" on cylinder head and #4 and #1 cylinder seem to be at the same height from cylinder head. Timing belt does not appear to have slipped is in good shape not worn, tension ok. Motor cranks over normally and spark plugs are wet. In December we had the O2 sensor, exhaust and catalytic converter worked on. It ran for about a week then quit. Please offer us some suggestions.