Hipp - clarification here. If you only have (for example) 4 sectors open and all are infiltrated then you need to conquer the province if that province is named. If you have a province w/more than 4 sectors in various states, but 4 sectors not infiltrated, then yes, infiltrate 4 will complete. Sergione has only 4 infiltrated sectors left on this province, so he is 'stuck'.
Test - I believe you are wrong. If he has additional provinces, conquer may not complete this quest. Sergione may need to scout/infiltrate a new province. And, as it is NOT clear if he is at end of map, Hipp is right. More info may be needed. But by all means, send a ticket. Let's see . . mk