Recall that (4) says that the party ought not to bear risk of the mistake.
Doesn't it follow from that alone that the contract should be voidable? After all, if the party really ought not to bear the risk, how can it be correct to enforce the contract? Enforcing the contract makes the party bear the risk.
The solution: (4) means that the party ought not to bear the risk as determined by the criteria summarized in Restatement (Second) Contracts § 154. What (5) does is just add one more requirement for voidability to the requirements summarized in § 154.
The following question and answer sequence works through Elsinore School Dist. v. Kastorff to illustrate the rule.