While surfing Wikipedia and various other Winx sites, I was able to figure out the most probable release dates for Winx Club Season 4. Then my predictions became more certain as I found a site (sorry, I forgot what it is :P) that said the release dates would be around then.
Here they are:
The RAI English version of Season 4 will probably release around August/September 2010. This is because they will need to record the voices, then add them to the video, and do all the various voice/video editing. They will also need to record the english transformation songs and themes.
Also, with Season 1, 2 and 3, the RAI english version released about 6 months to one year after the Italian release so it'll probably be the same for Season 4.
The 4Kids Season 4 will probably release later (like around November/December 2010) because they don't have to just do what RAI does, they cut some scenes and change the storyline around a little bit to make it more appropriate for kids (and we all know the amount of romantic drama in Season 4 ;)) They'll also probably cut the Believix, Sophiex and Lovix transformations like they did with Enchantix and replace the music with an instrumental.
PS: In Dubai, the Secret of the Lost Kingdom is releasing next week in cinemas! I don't know whether this is worldwide...