TOKYO, Jan 12 (Reuters) - Japan"s service sector sentiment index rose 0.1 point to 56.4 in December, a Cabinet Office survey showed on Wednesday, as domestic COVID-19 cases remained low through the month despite concerns around the new Omicron variant. That marked the fourth straight month of increase. The index, which was at 56.3 in November, is based on a survey of workers such as taxi drivers, hotel workers and restaurant staff who are called "economy watchers" for their proximity to consumer and retail trends.