According to a forum post on the Skyforge forum, it is very possible that open beta will hit sometime in July. According to Community Manager 'Spunky', there will be one more closed beta event prior to the commencement of the open beta.
Moderator Alanor wrote:
We've not received any official information on the subject. With the timeline we currently have of CBT4 this month and OBT sometime in Summer, I'd place my bet on OBT coming after CBT4 instead of a CBT5. Keep an eye on the News section for more official information on future beta tests.
Community Manager Spunky responded to the above by saying:
Barring any unforeseen complications, CBT4 will be the last CBT event before OBT. That being said, the time difference between CBT4 and OBT will depend on how well CBT4 goes.
Read more of the forum discussion at the link above.
Thanks to Wraithclaws for the tip!