As a non-Japanese-teacher, I think neither form is immediately wrong, or I can rather say that it mostly depends on personal preferences (disclaimer: I don't know if JLPT says otherwise :P).
Etymologically, the counter 羽【わ】 was は in its older form, obviously cognate with 羽【はね】 "wing". Unlike most counters, this short piece of word was easily affected by the phonological change that also impacted the pronunciation of topic particle は. Therefore, there is a disagreement among today's speakers on whether the base form of the counter is わ or は.
||よんわ / よわ (rare)
||よんわ / よんば†
||じっぱ / じゅっぱ‡
Speakers can freely choose either series, or even mix them as they like, and it does not hinder communication.
Similarly we have another counter 把【わ】 "bundle of" in the exactly same situation, where NHKアクセント新辞典 only lists ～わ except 十把 じっぱ/じゅっぱ for some reason, so this seems really a matter of discretion.
† よん + h- usually does not trigger rendaku, because よん is originally a replacement of native よ- or on'yomi し. So this is actually irregular.
‡ Japanese teachers may not be pleased.