What is sleep phase 4

[KTibow]

Expected behavior:

Each line in this file has the format: date (in seconds since PSG start) stage (0-5, wake = 0, N1 = 1, N2 = 2, N3 = 3, REM = 5)

Actual behavior:
some phases are marked as "4" in 5383425 (which seems to be another form of deep sleep)