x is an integer, and a,b,c,d distinct integers, so x-a,x-b,x-c,x-d are distinct integer factors of 4, with product 4.
A quick consideration of cases tells you these factors must be 2,-2,1,-1.
Then sum the 4 equations to get 4x-(a+b+c+d)=2-2+1-1=0, and you are done.
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