What needs to be clarified are the following four points.
Literal
Clarify must [nominalizer-の] [topic-は] next [possessive-の] 4-points [copula-である].
A formal written-Japanese construction using の to nominalize the whole left clause: 'that which must be clarified.' Then である (formal copula) closes the sentence. Typical of academic or technical writing where the writer introduces a list of points.