From their website:
The next expansion for Monsterpocalypse, Series 5: Big In Japan has been delayed due to factory and shipping delays due to increased production demand overseas. Big In Japan which was scheduled to release this April has been moved back to a May release. More information will be released as it becomes available on the product’s new release date.
In order to help with the gap between series, Privateer Press will be releasing two additional free Monsterpocalypse prize support kits available for stores to register and run on www.privateerevents.com. These kits include the final two monster mega forms from Series 4: Monsterpocalypse Now, Mega Aquosia and Mega Mucustos. Each kit will contain two copies of the monster mega form and a random assortment of shadow and glass promotional units equal to the number of pre-registered players.