Since Stallone has started his preparation for Rambo 5 - to be filmed in Bulgaria - the news outlets in the country have begun reporting on the star and his future trips to Bulgaria. 24 Chasa, one of the most reputable news outlets there is reporting that EX4 will indeed begin filming in Bulgaria in April.
Other semi-confirmation is Gregory Poirier's twitter page where he's now listing EX4 as one of his current projects. It was reported in the past that Poirier would team up with Sly to write the script for the fourth installment.