gdal - Reprojecting layer in PyQGIS?

I have a problem. I'm currently trying to get the extent of a vector layer in latitude and longitude coordinates in python. The layer is currently in EPSG:3857, so .fullExtent() is not returning what I want. I have tried reprojecting the layer using this:

self.layer= QgsVectorLayer(self.input, self.featureName, "ogr")
self.layer.setCoordinateSystem("crs=epsg:4326")

But the second line introduces a type error when loading the vector layer.

So what would be the best way of getting the bounds in latitude and longitude coordinates? It would be nice if I could get the coordinates from the pyqgis API, but I would be happy if there was another way tool that would do it, such as gdal.

Answer

my solution was:

canvas = qgis.utils.iface.mapCanvas()
allLayers = canvas.layers()
exp_crs = QgsCoordinateReferenceSystem(4326, QgsCoordinateReferenceSystem.EpsgCrsId)
for i in allLayers: 

    if i.type() != 0 :

        print(i.name() + " skipped as it is not a vector layer")  
    if i.type() == 0 :
        #exp_crs = QgsCoordinateReferenceSystem(4326,QgsCoordinateReferenceSystem.PostgisCrsId)
        print i.crs()
        print exp_crs
        qgis.core.QgsVectorFileWriter.writeAsVectorFormat(i, i.name() + '.js', 'utf-8', exp_crs, 'GeoJSON')

this saves all layers in a new projection