23) B

Let the first digit be A
Let the second digit be B
Let the third digit be C
Let the fourth digit be D

B – A = 1
D – C = 1
A + D = BC

We know, B – A = D – C = 1
B + C = A + D

We also know A + D = BC

So,
A + D = B + C = BC
BC = B + C

There is only one possibility which satisfies BC = B + C. That is if both numbers are 2.
2 + 2 = 4
2 x 2 = 4

So B and C are 2.
A must be 1.
D must be 3.

So the code is 1223

The fourth digit is 3.