Mashima has just tweeted that a 5th ova is coming and it will be based on chapter 298 of the manga. here's the tweet in Japanese, link. More details will be coming on the 36th volume of the manga which will be released on the 15th of February. This was pretty expected because the 298th chapter contains scenes that will probably be censored on regular TV or even removed. So it's unsurprising to see them do what they did with the 4th OVA .