Two things:
First possible is the vac line under the front engine cover may be cracked. This line is plastic and they dry out. It goes to the AIR switch-over-valve. Common fault, easy remedy.
Second , is a possible fading 02 sensor..
The sensor looks for a lean condition when the pump is energized
and if it does not see it , it pops a code 4. This lean condition requires a sensitive sensor as the voltage requirement is very low to trigger the test .
So, a slow/dirty sensor will not verify the test and the CE will light.
A good run on the highway in 3rd gear sometimes will clear up a dirty/carboned sensor.
This is assuming the AIR pump is coming on, of course...
If not , check the fuse..