/**
* Lazily Map 4th element and memoise the result
*
* @param fn Map function
* @return
*/
default <T> PTuple8<T1, T2, T3, T, T5, T6, T7, T8> lazyMap4(Function<T4, T> fn) {
if (arity() != 8) return (PTuple8) PTuple7.super.lazyMap4(fn);
return new LazyMap4PTuple8(fn, (PTuple8) this);
}
