They include an airline worker stopped from wearing a cross and a registrar who did not want to marry gay couples.
All four lost separate employment tribunals relating to their beliefs.
Secular critics have said a ruling in favour of the group could "seriously undermine" UK equality law.
A ruling is not expected from the European court for several weeks.
The cases involve:
- Nadia Eweida, a Pentecostal Christian from Twickenham, south-west London, who was sent home by her employer British Airways in 2006 after refusing to remove a necklace with a cross
- Devon-based nurse Shirley Chaplin, who was moved to a desk job by Royal Devon and Exeter NHS Trust Hospital for similar reasons
- Gary McFarlane, a Bristol relationship counsellor, who was sacked by Relate after saying on a training course he might have had a conscientious objection to giving sex therapy advice to gay couples
- Registrar Lilian Ladele, who was disciplined after she refused to conduct same-sex civil partnership ceremonies in north London