How would you do it without a heap allocation?

Since the OP wanted to return an array, the correct answer would probably be to return a simple native array, not a complex stdlib container:

    pub fn get_array() -> [i32;4] {
        return [1, 2, 3, 4];
    }

In C one needs to wrap the array in a struct type to work around the pointer decay:

    typedef struct {
        int items[4];
    } IntArray4;

    IntArray4 get_array(void) {
        return (IntArray4) {
            .items = { 1, 2, 3, 4 }
        };
    }

Both solution don't use heap allocation but pass around the array data by value on the stack (or in registers). That's one of the few cases where Rust code actually turns out more simple than the C equivalent ;)