A nice song from SWAT 4 - The Stetchkov Syndicate.

This song appears in the mission "Sisters of Mercy Hostel", during interior exploration.
At around 6:59, the song changes to that heard when you're inside the "Chapel" room.

It's essentially 'Ambient'.
Made by Eric Brosius, the same person who did songs, music and ambient for Thief, Thief II, Thief Deadly Shadows, System Shock 2 and various more.

Ambient songs are generally longer with a more obvious 'loop' sound to it.