To address the CWE-401 vulnerability at line 4, we need to ensure that the `page` pointer is not used after it has been freed. Here are five possible patches:

**Patch 1: Check if page is NULL after free_unref_page**
```c
if (pcp_allowed_order(order)) { /* Via pcp? */
    free_unref_page(page, order);
    if (page != NULL) {
        // Handle the case where page is not NULL after free_unref_page
        // This could indicate a double-free or other memory corruption issue
        printk(KERN_ERR "Page not freed: %p\n", page);
    }
} else
    __free_pages_ok(page, order, FPI_NONE);
```

**Patch 2: Set page to NULL after free_unref_page**
```c
if (pcp_allowed_order(order)) { /* Via pcp? */
    free_unref_page(page, order);
    page = NULL; // Set page to NULL to prevent further use
} else
    __free_pages_ok(page, order, FPI_NONE);
```

**Patch 3: Use a temporary pointer for free_unref_page**
```c
if (pcp_allowed_order(order)) { /* Via pcp? */
    struct page *tmp_page = page;
    free_unref_page(tmp_page, order);
} else
    __free_pages_ok(page, order, FPI_NONE);
```

**Patch 4: Check if page is valid before calling free_unref_page**
```c
if (pcp_allowed_order(order)) { /* Via pcp? */
    if (page != NULL && page_ref_count(page) > 0) {
        free_unref_page(page, order);
    } else {
        // Handle the case where page is NULL or has no references
        printk(KERN_ERR "Invalid page: %p\n", page);
    }
} else
    __free_pages_ok(page, order, FPI_NONE);
```

**Patch 5: Add a page reference before calling free_unref_page**
```c
if (pcp_allowed_order(order)) { /* Via pcp? */
    get_page(page); // Add a reference to the page
    free_unref_page(page, order);
    put_page(page); // Release the reference
} else
    __free_pages_ok(page, order, FPI_NONE);
```

Note that these patches are just suggestions and may require additional modifications to ensure correctness and compatibility with the surrounding code. A thorough review and testing of the patches are necessary to ensure that they effectively address the CWE-401 vulnerability.