Who knows why this Sudoku has level-4 ?

Today Metro page 23 (12 second):

Sudoku Line
XXXX5X2XX
X2XXX7X83
586XXXXXX
XX36XXXX7
XX2XXX1XX
8XXX1X4XX
XXXXXX395
46X5XXX2X
XX8X9XXXX

Line 1: | 7 | 3 | 1 | 9 | 5 | 8 | 2 | 4 | 6 |
Line 2: | 9 | 2 | 4 | 1 | 6 | 7 | 5 | 8 | 3 |
Line 3: | 5 | 8 | 6 | 2 | 4 | 3 | 9 | 7 | 1 |
Line 4: | 1 | 9 | 3 | 6 | 2 | 4 | 8 | 5 | 7 |
Line 5: | 6 | 4 | 2 | 8 | 7 | 5 | 1 | 3 | 9 |
Line 6: | 8 | 7 | 5 | 3 | 1 | 9 | 4 | 6 | 2 |
Line 7: | 2 | 1 | 7 | 4 | 8 | 6 | 3 | 9 | 5 |
Line 8: | 4 | 6 | 9 | 5 | 3 | 1 | 7 | 2 | 8 |
Line 9: | 3 | 5 | 8 | 7 | 9 | 2 | 6 | 1 | 4 |

Lenny