The grn wire is 02 sig feed to ECU, so the other side of that plug is 02 sensor output.
The two whites are heater [12v.] and the other is ground.
You want to use the ground wire for neg. lead when testing 02 sens output
b/c they do not use chassis ground on 4 wire sensor with HFM/SFI systems...
best place to catch that neg is at terminal # 3 of the 3 prong connector..that will be the blk coming out of that connector...