In this case a[4] is the 5th integer in the array a, ap is a pointer to integer, so you are assigning an integer to a pointer and that’s the warning.
So ap now holds 45 and when you try to de-reference it (by doing *ap) you are trying to access a memory at address 45, which is an invalid address, so your program crashes.
You should do ap = &(a[4]); or ap = a + 4;
In c array names decays to pointer, so a points to the 1st element of the array.
In this way, a is equivalent to &(a[0]).