AIR code [4] is common malfaunction of the AIR Injection System on HFM/104s...
This is an emissions related code , as the AIR is called upon to heat the cat rapidly at cold start to get the car into closed loop as fast as possible .

So, when diagnosing,we seperate the system into 2 catagories... one is electrical and the other is vac/air system.
If the pump comes ON at start-up, we do not have an electrical feed problem ..So we go into the air passage system.
But , if the pump does not come on , we start with 'Why not?'
First check is fuse.
If the pump does come on, then the first suspect on that model is the white plastic vac line running across the front of the engine , over to the EGR and AIR Switch-Over-Vaves. They crack/fatique from engine heat and shouild be replaced with a full rubber line.
**This line is COMMON vac feed for BOTH EGR and AIR SOVs , so it may well also be your EGR code cause !! When these two codes come up, this line is first suspect......
The next suspect is the SOV and check valve, but check the above first so I don't load your plate up all at once.