Solution by Quine-McCluskey
Numbers are partially attached to show what and what are compbined.

(1) 0 1 1 1    1,4  0 1 1 -        0 - 1 -  (*)
(2) 1 0 1 1    1,5  0 1 - 1        0 1 - -  (*)
-----------         0 - 1 1        0 - - 0  (*)
(3) 1 0 0 1         1 0 - 1 (*)
(4) 0 1 1 0         - 0 1 1 (*)
(5) 0 1 0 1         -------
(6) 0 0 1 1    4,7  0 1 - 0
-----------    4,8  0 - 1 0
(7) 0 1 0 0         0 1 0 -
(8) 0 0 1 0         0 0 1 -
-----------         -------
(9) 0 0 0 0         0 - 0 0
               8,9  0 0 - 0

Terms with (*) must be considered for the minimaal form.

             0111  1011  1001  0110  0101  0011  0100  0010  0000
-------------------------------------------------------------------
1 0 - 1 * |          x     x
- 0 1 1   |          x                       x
0 - 1 -   |    x                 x           x           x
0 1 - - * |    x                 x     x           x
0 - - 0 * |                      x                 x     x     x
-------------------------------------------------------------------
 
Essential terms are with *. To cover 0011 0-1- is cheaper than -011.
Thus the solution is x1'x3+x1'x2+x1'x4'+x1x2'x4