Inconsistency in returning primitive arrays in Java

The following show different ways of instantiating and returning a primitive array. However, for some reason, the last one doesn't work. Is there a valid explanation for this inconsistency? Why doesn't the last block work?

Block 1

    int[] a = new int[] {50};
    return a;    // works fine

Block 2

    int[] a = {50};
    return a;    // works fine

Block 3

    return new int[] {50};    // works fine

Block 4

    return {50};   // doesn't work