Try:
CGFloat myScale = 4.0;
[view setContentScaleFactor:myScale*[[UIScreen mainScreen] scale]]];
Additionally, if you were embedding your view within a UIScrollView to allow the user to zoom in/out you could put this implementation in your UIScrollViewDelegate:
- (void)scrollViewDidEndZooming:(UIScrollView *)scrollView withView:(UIView *)view atScale:(float)scale {
scale *= [[[scrollView window] screen] scale];
[view setContentScaleFactor:scale];
}