Regular languages and sets proof
Always think of boundary cases. In this case - the languages that contain every string or no string. 

Also, let $N$ be some arbitrary non-regular language.

i) The language that contains no strings, which is regular, is a subset of $N$.

ii) $N$ is a subset of the language that contains all strings, which is regular.

iv) For every string $S_i$ in $N$, construct a set $L_i$ that contains only $S_i$. Note that all of these constructed sets are trivially regular. But the union of all of these (infinite number of) constructed sets is $N$, which is not regular.