That Georgie Henley Twitter account is almost definitely a fake. Firstly, her character has very little involvement in the remaining stories so Georgie doesn't and wouldn't know what's going on with future productions. Secondly, there are currently big contractual problems with the Narnia series which means that the earliest production on a fourth film could begin is 2014 and if it's made by a studio other than Walden Media (the studio behind the previous films) then it cannot be released until at least 2018.
Rumours suggest that Walden Media probably won't be involved with the future films because the C.S. Lewis estate are not happy with the direction of the films. It's therefore unlikely that production on the next film will begin any time soon.