If you are clever, the thief needs 49 different settings of the dials to know the correct setting with certainty. This is more than half of all $90 = 4\cdot9 + 3\cdot9 + 2\cdot9 + 1\cdot9$ possible settings you can produce (when moving one dial, two adjacent dials, three adjacent dials, and all four dials, respectively, into their nine possible incorrect positions).
Let the dials be $(a, b, c, d)$. Let the correct position be $(0, 0, 0, 0)$ to have a mental picture. If you put $a$ in eight different positions, say 1, 2, ..., 8, then the thief does not yet know with certainty the correct one - although she knows the correct positions $0$ of the other three. So if you decided to turn one or more other dials leaving $a$ at $0$, she would quickly know the complete correct setting. But if you put $(a, b)$, $(a,b,c)$, and $(a,b,c,d)$ also in the eight different positions avoiding $a = 9$, you have $4\cdot8 = 32$ positions without revealing the correct information.
You can do even better, if you choose to start with $b$. Then you can extend your number of settings by moving $(a,b)$, $(b,c)$, $(a,b,c)$, $(b,c,d)$, and $(a,b,c,d)$ supplying $(1 + 2 + 2 + 1)\cdot8 = 48$ settings in total. (Of course the order you choose does not matter.)
You would get the same opportunity with $c$ instead of $b$. $d$ however, like $a$, would supply only 32 possible positions.
The maximum number of different positions, before the thief has discoverd the correct one, is for $n > 2$ digits and an even number $m$ of dials:
For an odd number $m$ of dials you get
If no single dial is moved, we have only 48 settings: 3 pairs, 2 triples and 1 quadruple in 8 positions each. But if pair $(a,b)$ has been moved twice, pair $(c,d)$ cannot be moved without revealing the secret. Hence, we get only 40 settings. That is less than the constructed 48. So, in order to maximize the number of the secret-maintaining settings, we have to move also at least one single dial. But having moved it twice, we can no longer move any other single dial or the pair not containing the first. This subtracts 36 from the 90 possible settings. Since of the remaining 54 settings 6 are always "the nineth", i.e., revealing the secret, we have at most 48 settings.